Geognosy.
The constitution of the earth.—Turning from its external relations to the earth itself, a natural threefold division is presented: (1) the atmosphere, (2) the hydrosphere, and (3) the lithosphere.
I. The Atmosphere.
The atmosphere is an intimate mixture of (1) all those substances that cannot take a liquid or solid state at the temperatures and pressures which prevail at the earth’s surface, together with (2) such transient vapors as the various substances of the earth throw off. The first class form the permanent gases of the atmosphere, and consist of nitrogen about 79 parts, oxygen about 21 parts, carbon dioxide about .03 part, together with small quantities of argon, neon, xenon, krypton, helium, and other rare constituents. The second class are the transient and fluctuating constituents of the atmosphere, chief among which is aqueous vapor, which varies greatly in amount according to temperature, pressure, and other conditions. To this are to be added volcanic emanations and a great variety of volatile organic substances. Theoretically, every substance, however solid, discharges particles which may transiently become constituents of the atmosphere. Practically, only a few of these exist in such quantity as to be appreciable. Dust and other suspended matter are usually regarded as impurities rather than constituents of the atmosphere, but they play a not unimportant part by affecting its temperature and luminosity, and by facilitating the condensation of moisture.
Mass and extent.—The total mass of the atmosphere is estimated at five quadrillion tons, or ¹⁄₁₂₀₀₀₀₀ of the mass of the earth. It is relatively dense at the surface of the earth and decreases in density outwards in a manner difficult of absolute determination, so that the actual height of the appreciable atmosphere is not positively known. The true conception of the atmosphere is perhaps that of a tenuous envelope exerting a pressure of about fifteen pounds per square inch at the sea-level, and thinning gradually upwards until it reaches a tenuity which is inappreciable, but perhaps not ceasing absolutely until the sphere of gravitative control of the earth is passed, about 620,000 miles from the lithosphere. In the lower portion, according to the kinetic theory of gases, the molecules fly to and fro, colliding with each other with almost inconceivable frequency, and with very short paths between successive collisions, but in the upper rare portion some of the molecules bound outwards, and do not strike other molecules, and hence pursue long elliptical paths until the gravity of the earth overcomes their momentum, when they return, perhaps to bound off again or to force other molecules to do so. This fountain-like nature of the outer part of the atmosphere makes any sharp definition of its limit impracticable. Some molecules are believed to be shot away at such speed that they do not return. Beyond about 620,000 miles from the surface of the lithosphere, the differential attraction of the sun is greater than that of the earth, and if the attraction of the earth does not turn the molecules back before reaching this distance, they are almost certain to be lost to the earth.
The measurement of heights by the aneroid barometer, which is much used in practical geology, is dependent on the lessening of pressure as the instrument is carried upward.
Geologic activity.—The atmosphere is the most mobile and active of the three great subdivisions of the earth, and when its indirect effects through the agency of water, as well as its direct effects, are considered, it is to be regarded as one of the most effective agencies of change. It acts chemically upon the rock substance of the earth, causing induration in some instances, but more often inducing disintegration and change of composition by means of which rock is reduced to soil, or soil-like material, and rendered susceptible of easy removal by winds and waters. When in motion the atmosphere acts mechanically on the surface of the earth, transporting dust and sand, and by the friction of these it abrades the surface. It is chiefly effective, however, in furnishing the conditions for water action. Partly by its mechanical aid, but chiefly by securing the right temperature, it is a necessary factor in the action of rains, streams, glaciers, and the various forms of moving water upon land. So also, on the ocean, wave action is essentially dependent on the winds. In the absence of atmospheric propulsion, wave action would be chiefly confined to the tides and to occasional earthquake impulses, and would lose nearly all its efficiency. Stream action and wave action, which are the most declared of the geological agencies, are therefore to be credited as much to the atmosphere as to the hydrosphere, since the action is a joint one to which both envelopes are essential.
A thermal blanket.—A function of the atmosphere of supreme importance is the thermal blanketing of the earth. In its absence the heat of the sun would reach the surface with full intensity, and would be radiated back from the surface almost as rapidly as received, and only a transient heating would result. During the night an intensity of cold would intervene scarcely less severe than the temperature of space. In penetrating the atmosphere certain portions of the radiant energy of the sun are absorbed. Of the remainder which reaches the surface of the earth, a part is transformed into vibrations of lower intensity, which are then more effectively retained by the atmosphere. The air thus distributes and equalizes the temperature. The two constituents of the atmosphere which are most efficient in this work are aqueous vapor and carbon dioxide, and the climate of the earth is believed to have been very greatly affected by the varying amounts of these constituents in the atmosphere, as well as by the total mass of the atmosphere.
The function of the atmosphere in sustaining life and promoting all that depends on life is too obvious to need comment.
The special geological action of the atmosphere will be discussed in the next chapter.
II. The Hydrosphere.
About 1300 quadrillion tons of water lie upon the surface of the solid earth. This equals about ¹⁄₄₅₄₀ part of the earth’s mass. Were the surface of the solid earth perfectly spheroidal, this would constitute a universal ocean somewhat less than two miles deep. Owing to the inequalities of the rock surface, the water is chiefly gathered into a series of great basins or troughs occupying about three-fourths (72%) of the earth’s surface. These basins are all connected with each other and act as a unit, so that anything which changes the level of the water in one changes the level of all. This helps to make a common record of all great movements of the earth’s body, for the level of the ocean determines where the detritus from the land shall lodge, and hence where the edge of the marine beds shall be formed. This will appear more clearly when the formation of marine strata is discussed.
Oceanic dimensions.—The surface area of the ocean is estimated by Murray at 143,259,300 square miles. Of this, somewhat more than 10,000,000 square miles lie on the continental shelf, i.e., lap up on the borders of the continental platforms. This shows that the great basins are somewhat more than full. If about 600 feet of the upper part of the ocean were removed, the true ocean basins would be just full, and the surfaces of the true continental platforms would be dry land. The area of the true oceanic basins is about 133,000,000 square miles, and that of the true continental platforms about 64,000,000 square miles. Under about 20% of the ocean area, the bottom sinks to depths between 6000 and 12,000 feet; under about 53% it sinks to depths between 12,000 and 18,000 feet; and under the remaining 4% it ranges from 18,000 feet down to about 30,000. The last includes those singular sunken areas known as “deeps,” and sometimes called anti-plateaus, as they extend downward from the general ocean bottom much as the plateaus protrude upwards from the general land surface.
Besides the ocean, the hydrosphere includes all the water which constitutes the surface streams and lakes, together with that which permeates the pores and fissures of the outer part of the solid earth; but altogether these are small in amount compared with the great ocean mass.
Geologic activity.—Of all geological agencies water is the most obvious and apparently the greatest, though its efficiency is conditioned upon the presence of the atmosphere, upon the relief of the land, and upon the radiant energy of the sun. Through the agency of rainfall, of surface streams, of underground waters, and of wave action, the hydrosphere is constantly modifying the surface of the lithosphere, while at the same time it is bearing into the various basins the wash of the land and depositing it in stratified beds. It thereby becomes the great agency for the degradation of the land and the building up of the basin bottoms. It works upon the land partly by dissolving soluble portions of the rock substance, and partly by mechanical action. The solution of the soluble part usually loosens the insoluble, and renders it an easy prey of the surface waters. These transport the loosened material to the valleys and at length to the great basins, meanwhile rolling and grinding it and thus reducing it to rounder forms and a finer state, until at length it reaches the still waters or the low gradients of the basins and comes to rest. The hydrosphere is therefore both destructive and constructive in its action. As the beds of sediment which it lays down follow one another in orderly succession, each later one lying above each earlier one, they form a time record. And as relics of the life of each age become more or less imbedded in these sediments, they furnish the means of following the history of life from age to age. The historical record of geology is therefore very largely dependent upon the fact that the waters have thus buried in systematic order the successive life of the ages. Aside from this, the means of determining the order of events of the earth’s history are limited and more or less uncertain.
The special processes of the hydrosphere in its various phases will be the subject of discussion hereafter (Chaps. [III], [IV], [VI]). Suffice it here to recognize its great function in the constant degradation of the land, and in the deposition of the derived material in orderly succession in the basins.
Chief horizons of activity.—The great horizons of geological activity are (1) the contact zone between the atmosphere and the hydrosphere, chiefly the surface of the ocean, (2) the contact zone between the hydrosphere and the lithosphere, chiefly the shore belts, and (3) the contact zone of the atmosphere and surface waters, with the face of the continents. It is in these three zones that the greatest external work is being done and has been done in all the known ages.
III. The Lithosphere.
The atmosphere and hydrosphere are rather envelopes or shells than true spheres, though in some degree both penetrate the lithosphere. The lithosphere, on the other hand, is a nearly perfect oblate spheroid with a polar diameter of 7899.7 miles, and an equatorial diameter of about 26.8 miles more. Its equatorial circumference is 24,902 miles, its meridional circumference 24,860 miles, its surface area 196,940,700 square miles, its volume 260,000,000,000 cubic miles, and its average specific gravity about 5.57. The oblateness of the spheroid is an accommodation to the rotation of the earth, the centrifugal force at the equator being sufficient to cause the specified amount of bulging there. Computations seem to indicate that the accommodation is very nearly what would take place if the earth were in a liquid condition, from which the inference has been drawn that it must have been in that condition when it assumed this form, and must have continued essentially liquid until it attained its present rate of rotation, since, if the earth once rotated at a much higher speed, the flattening at the poles and the bulging at the equator must have been correspondingly greater. It is thought by others, however, that the plasticity of the earth is such that it would at all times assume a close degree of approximation to the demands of rotation, even if the interior were in a solid condition. By still others it is thought that the contraction of the earth has tended to accelerate the rotation about as much as the tides have tended to retard it, and that it has undergone little change of form.
Irregularities.—It is only in a general view, however, that there is a close approximation to a perfect spheroidal surface. In detail there are very notable variations from it. Geodetic surveys seem to have shown that the equatorial diameters are not all equal, even when the measurements are reduced to sea-level, but research along this line has not reached a sufficient stage of completeness to permit satisfactory discussion. It is, however, highly probable that the ocean surface as well as the average land surface is warped out of the perfect spheroidal form to some notable degree. This is very likely due to inequalities in the density of the earth’s interior. The fact that the larger portion of the water is gathered on one side of the globe, while the land chiefly protrudes on the opposite side, is very possibly due to unequal specific gravity in the interior of the earth.
The most obvious departure from a spheroidal form is found in the protrusion of the continents and in the sinking away of the earth surface under the oceans. As these inequalities present themselves to-day, they are known as continental platforms and ocean basins. These do not correspond accurately with the present land and water surfaces. About the continental lands there is a submerged border extending some distance out from the shore, and constituting a sea-shelf beyond which the surface descends rapidly to the great depths of the ocean. This slightly submerged portion, known as the continental shelf, belongs as properly to the continent as the adjacent low lands which are not submerged. The submergence of the edge of this shelf at present is usually about 100 fathoms, so that if the upper 600 feet of the ocean were removed, the outlines of the land would correspond quite closely with the border of the true continental platform.
BATHYMETRICAL CHART OF THE OCEANS
SHOWING THE “DEEPS” ACCORDING TO SIR JOHN MURRAY
It is customary to look upon the protrusions of the continents as the great features of the earth’s surface, but in reality the oceanic depressions are the master phenomena. In breadth, depth, and capacity they much exceed the continental protrusions, and if the earth be regarded as a shrunken body, the settling of the ocean bottoms has doubtless constituted its greatest surface movement. From the estimates of Murray, Gilbert has derived the following tables, showing the relative areas of the lithosphere above, below, and between certain levels.[1]
From these estimates it appears that if the surface were graded to a common level by cutting away the continental platforms and dumping the matter in the abysmal basins, the average plane would lie somewhere near 9000 feet below the sea-level. The continental platform may be conceived as rising from this common plane rather than from the sea-level.
| Contours. | Percent. of Surface above. | Percent. of Surface below. |
|---|---|---|
| Contour 24,000 feet above sea-level | 0.004 | 99.996 |
| “ 18,000 “ “ “ | 0.09 | 99.91 |
| “ 12,000 “ “ “ | 0.7 | 99.3 |
| “ 6,000 “ “ “ | 2.3 | 97.7 |
| Sea level | 27.7 | 72.3 |
| Contour 6,000 feet below sea-level | 42.5 | 57.5 |
| “ 12,000 “ “ “ | 57.3 | 42.7 |
| “ 18,000 “ “ “ | 96.8 | 3.2 |
| “ 24,000 “ “ “ | 99.93 | 0.07 |
| Percent. | |
|---|---|
| More than 6000 feet above sea-level | 2.3 |
| Between sea-level and 6000 feet above | 25.5 |
| Between sea-level and 6000 feet below | 14.8 |
| Between 6000 and 12,000 feet below sea-level | 14.8 |
| Between 12,000 and 18,000 feet below sea-level | 39.4 |
| Between 18,000 feet and 24,000 feet | 3.1 |
Epicontinental seas.—Those shallow portions of the sea which lie upon the continental shelf, and those portions which extend into the interior of the continent with like shallow depths, such as the Baltic Sea and the Hudson Bay, may be called epicontinental seas, for they really lie upon the continent, or at least upon the continental platform; while those other detached bodies of water which occupy deep depressions in the surface are to be regarded as true abysmal seas, as, for example, the Mediterranean and Caribbean seas and the Gulf of Mexico, whose bottoms are as profound as many parts of the true ocean basin itself.
Diversities of surface.—The bottoms of the oceanic basins are diversified by broad undulations which range through many thousands of feet, but they are not carved into the diversified forms that give variety to land surfaces. The ocean bottoms are also diversified by volcanic peaks, many of which rise to the surface and constitute isolated islands. Some of them have notable platforms at or near the surface, cut by the waves or built up by the accumulation of sediment and of coralline and other growths about them. Aside from these encircling platforms, the solid surface usually shelves rapidly down to abysmal depths, so that the islands constitute peaks whose heights and slopes would seem extraordinary if the ocean were removed.
The surface of the land is diversified in a similar way by broad undulations and volcanic peaks, and also by narrower wrinklings and foldings of the crust; but all of these irregularities have been carved into diversified and picturesque forms by subaërial erosion. In this respect the surface of the land differs radically from the bed of the sea. The agencies which have produced the continental platforms and abysmal basins, and the great undulations and foldings, as well as the volcanic extrusions that mark them, are yet subjects of debate. Here lie some of the most difficult problems of geology, but these cannot be stated with sufficient brevity to find a place here.
The surface mantle of the lithosphere.—The surface of the lithosphere is very generally mantled by a layer of loose material composed of soil, clay, sand, gravel, and broken rock. This loose material is sometimes known as mantle rock, and sometimes as rock waste. On the land, mantle rock is often composed of the disintegrated products of underlying rock formations. It represents the results of the recent action of the atmosphere, of water, of changes of temperature, and of other physical agencies acting on the outer part of the rock sphere. The surface of this mantle is being constantly removed by wind and water, but as constantly renewed by continued decomposition of the rock below. In some areas, especially in the northern part of North America and the northwestern part of Europe, the soil graduates down into an irregular sheet of mixed clay, sand, gravel, and bowlders, known as drift. From this and other evidence it is inferred that at a time not greatly antedating our own, ice, chiefly in the form of glaciers, spread extensively over the high latitudes of the northern hemisphere. In some parts of the earth the surface is still covered by fields of snow and ice, comparable to those which formed the drift. In still other places, especially along the flood plains of streams, the mantle rock consists of deposits made by streams which were unable to carry their loads of sediment to the sea.
The crust of the lithosphere.—Much of the detritus washed down from the land finds its way to bodies of standing water, and beneath lakes and seas the mantle of loose material is made up largely of the gravel, sand, and mud derived from the land. Before deposition these materials are more or less assorted and arranged in layers by waves and currents. When consolidated they constitute rock. The weathering of the rocks of the land, the wearing away of the resulting detritus, and its deposition beneath standing water, are among the most important processes of geologic change.
On the land, the mantle of loose material is sometimes absent, and in such places the surface of solid rock of the crust appears. Bare surfaces of rock are most commonly seen where the topography is rough, especially on the slopes of steep-sided valleys and mountains, and on the slopes of cliffs which face seas or lakes. Solid rock, without covering of soil or loose material of any sort, is also frequently seen in the channels of streams, especially where there are falls or rapids.
We have but to note the effects of a vigorous shower on a steep slope, or of a swift stream on its channel, or of waves on the cliffs which face lakes and seas, to understand at least one of the reasons why loose materials are frequently absent from steep slopes. The very general exposure of solid rock where conditions favor surface erosion suggests that rock is everywhere present beneath the soil or subsoil. Fortunately there is an easy way of testing the universality of the crust beneath the mantle. In all lands inhabited by civilized peoples there are numerous wells and other excavations ranging from a few feet to several hundred feet in depth, and occasional wells and mine-shafts reach depths of several thousand feet. Even in shallow excavations rock is often encountered, and in most regions excavations as much as two or three hundred feet deep usually reach rock, and no really deep boring has ever failed to find it. It may, therefore, be accepted as a fact that the upper surface of the solid rock is nowhere far below the surface.
Concerning the thickness of the crust, if there be any true crust at all, little is known by direct observation. The deepest valleys, such as the canyon of the Colorado, and the shafts and borings of the deepest mines and wells, give knowledge of nothing but rock. The deepest excavations extend rarely more than a mile below the surface. It is certain that rock of known kinds extends to far greater depths.
The interior.—Concerning the great interior of the earth, little is known except by inference. From the weight of the earth,[2] it is inferred that its interior is much more dense than its surface. From its behavior under the attraction of other bodies, it is believed to be at least as rigid as steel, and its interior cannot, therefore, be liquid, in the usual sense of that term. From the phenomena of volcanoes, and from observations on temperature in deep borings, it is inferred that its interior is very hot. Further inferences concerning its character are less simply stated, and will be referred to later.
The solid part of the earth is therefore composed of (1) a thin layer of unconsolidated or earthy material, a few feet to a few hundred feet in thickness, covering (2) a layer or zone many thousands of feet, and probably many miles, thick, composed of solid rock comparable to that exposed at numerous points on the surface, and (3) a central mass, to which the preceding layers are but a shell, composed of hot, dense, and rigid rock, the real nature of which is not known by observation.
Varieties of rock in crust.—If the mantle of soil, subsoil, and glacial rubbish were stripped from the land, the surface beneath would be found to be made up of a great variety of rocks, all of which may be grouped into two great classes. About four-fifths of the land surface would be of rock arranged in layers, and the other fifth would be of crystalline rock, generally without distinct stratification, and often bearing evidence of the effects of high temperature.
Stratified rocks.—The composition of most stratified rocks corresponds somewhat closely with the composition of sediments now being carried from the land and being deposited in the sea. Their arrangement in layers is the same, and the markings on the surfaces of the layers, such as ripple-marks, rill-marks, wave-marks, etc., are identical. Furthermore, the stratified rocks of the land, like the recent sediments of the sea, frequently contain the shells and skeletons of animals, and sometimes the impressions of plants. Most of the relics of life found in the stratified rocks belonged to animals or plants which lived in salt water. Because of their structure, their composition, their distinctive markings, and the remains of life which they contain, it is confidently inferred that most of the stratified rocks which lie beneath the mantle rock of the land were originally laid down in beds beneath the sea, and that the familiar processes of the present time furnish the key to their history.
Fig. 1.—Beds of (Cambrian) sandstone, a, are conformable with one another, but unconformable on beds of (Huronian) quartzite, b, Near Ableman, Wis.
Conformability.—When the stratified rocks exposed by the removal of the mantle rock are examined, the successive beds are sometimes found to lie on one another in regular succession, showing that they were laid down one after another, without change in the attitude of the surface on which they were deposited. Such rocks are conformable (the beds of series a, [Fig. 1]). In other cases it would be seen that certain beds overlie the worn surfaces of lower beds, the layers of which may have a different angle of inclination (series a, [Fig. 1], is unconformable on series b). Such relations show that the lower series of beds was disturbed and eroded before the overlying beds were deposited on them. Such series of rocks are unconformable.
Relative ages.—The structure and relations of rocks lead to inferences as to their relative ages. In the case of stratified rocks it is obvious that overlying beds were deposited later than those below, and where there is unconformity it is evident that an interval of time elapsed between the deposition of the unconformable series. Another and in some respects more important means of telling their order of formation is found in the remains of life entrapped in the water-laid sediments. Whatever life existed in the waters in which the sediments were deposited was liable to burial, and if it was possessed of hard parts, such as bones, teeth, shells, hard integuments, etc., these parts, or at least their impressions, were likely to be preserved in the sediments. Even tracks and imprints of perishable parts are sometimes preserved. All these relics, which we call fossils, give indications of the kinds of life which existed when the beds were formed. The fossils of the youngest beds show that the life which existed when they were deposited was quite like that of the present time. The fossils of the next older and lower beds show greater departure from present types. This series of changes continues downward as lower and lower beds are studied, until beds at considerable depths contain no relics of existing species but, in lieu thereof, forms of more primitive types. Some of these earlier types are clearly the ancestors of more modern forms, while others seem to have no living descendants. Going still deeper, the fossils indicate life of more and more primitive types, until they depart very widely from the living forms, and seem to be but remotely ancestral. So the beds may be followed downward until the lowest, which contain distinct evidences of life, are reached.
It should be understood that it is not possible to proceed directly downward through the whole succession of bedded rocks, but that the edges of the various beds may be found here and there where they have been brought to the surface by warpings or tiltings, or exposed by the wearing away of the beds which once overlay them. The full series of strata is made out only by putting together the data gathered throughout all lands, and even when this is done an absolutely complete series cannot yet be made out or, at least, has not been.
The crystalline rocks.—The crystalline rocks which would appear if the mantle rock were removed are of two types, igneous and metamorphic. Igneous rocks may be loosely defined as hardened lavas. Metamorphic rocks are those which are greatly changed from their original condition. Either stratified or igneous rocks may become metamorphic.
Igneous rocks sustain various relations to the stratified rocks, as illustrated by [Fig. 2]. From these relations it is possible to tell something of the order of their formation. Where the stratified rocks are broken through by lavas, it is obvious that the stratified rocks were formed first, and the lavas intruded later. Lava sheets intruded between beds of stratified rock can be told from those which flowed out on the surface and were subsequently buried, for in the former case the sedimentary rocks, both above and below the igneous rock, were affected by the heat, while in the latter case only those below were so affected.
Fig. 2.—Diagrammatic representation of the relations of igneous rock to stratified rock. The igneous rocks, represented in black, have been forced up from beneath.
More commonly than otherwise the metamorphic rocks ([Fig. 3]) lie beneath the sedimentary beds and are often broken through by the igneous rocks. From their position in many places their great age may be inferred, but locally, especially where dynamic action has been severe, relatively young rocks are metamorphic.
Fig. 3. The figure represents a section of the earth about 1000 miles Long. The unequally thick black line at the top represents on something like its proper scale the depth of the stratified rock. The area below represents crystalline rock, largely metamorphic.
Four great sedimentary eras.—The water-laid series represents four great eras in the history of the earth, as shown by the relics of life imbedded in them. Beginning with the latest, these are the Cenozoic (recent life), during which the life took on its modern aspect; the Mesozoic (middle life), during which the life bore a mediæval aspect; the Paleozoic (ancient life), during which the life belonged to older types; and the Proterozoic (earlier life), during which it is inferred that much life prevailed, though its record is very imperfect. It may safely be assumed to have been more primitive than that of the Paleozoic, as it was earlier. Each of these great divisions embraced several lesser periods or epochs, and these again are subdivided more and more closely according to the degrees of refinement to which studies are carried. The chief of these subdivisions are given in the table on [page 19], and others will come under consideration in the historical chapters.
In these four great series of sedimentary rocks there are, here and there, intrusions of igneous rocks, and in some places the sedimentary beds have been metamorphosed into crystalline rocks by heat and pressure. This is particularly true in the lowest of these series, the Proterozoic, where a large part of the sediment is metamorphosed, and where there is much igneous rock, but it is still clear that the main portion of this series was originally water-laid sediment, and so it belongs to the sedimentary series rather than the Archean, in which the sediments are the minor rather than the main factor. It has, however, usually been classed with the Archean, and it is certainly not always easy to draw the dividing line. In a sense it may be regarded as a transition series.
The Archean complex.—Beneath the dominantly sedimentary but partly metamorphic and igneous series there is a very complex group of rocks largely of metamorphosed igneous origin, though containing some metamorphosed sediments. These extend downwards to unknown depths. While all the great formations are occasionally bent and broken, these lowest ones are almost everywhere warped, folded, and contorted, often in the most intricate way. They have been very generally mashed and sheared by enormous pressure, so that they have become foliated, and their original character is much masked. They therefore form a series of great obscurity and complexity. As they are at the bottom of the known series, they have been called the “Fundamental gneiss” and the “Basement complex,” but as the part which we see is not the true base nor the true foundation, it is safer to call them simply the Archean (very ancient) complex. As life appears to have been present during a part at least of the period of its formation is referred to the Archeozoic era.
Fig. 4.—Diagram to illustrate the relations of the five great groups of formations.
AR = Archean, Pr = Proterozoic, P = Paleozoic, M = Mesozoic, C = Cenozoic.
Beyond and below this series, the structure of the earth is a matter of inference. Vast as are the preceding series, they together form relatively but a thin shell on the outer surface of the globe.
The foregoing series are diagrammatically expressed in [Fig. 4], and systematically presented to the eye in the following table.
| Cenozoic |  | Present. | ||
| Pleistocene. | ||||
| Pliocene. | ||||
| Miocene. | ||||
| Oligocene. | ||||
| Eocene. | ||||
| Transition (Arapahoe and Denver). | ||||
| Mesozoic | | Upper Cretaceous. | ||
| Lower Cretaceous (Comanche or Shastan). | ||||
| Jurassic. | ||||
| Triassic. | ||||
| Paleozoic | | Permian. | ||
| Coal Measures or Pennsylvanian. | ||||
| Subcarboniferous, or Mississippian. | ||||
| Silurian. | ||||
| Devonian. | ||||
| Ordovician. | ||||
| Cambrian. | ||||
| Great interval. | ||||
| Proterozoic | | Keweenawan. | ||
| Interval. | ||||
| Animikean or Penokean. (Upper Huronian of some authors). | ||||
| Interval. | ||||
| Huronian. | ||||
| Great interval. | ||||
| Archeozoic | | Archean Complex. | | Great Granitoid Series. (Intrusive in the main, Laurentian.) |
| Great Schist Series. (Mona, Kitchi, Lower Keewatin, Coutchiching, Lower Huronian of some authors.) | ||||
The purpose of this general survey is to bring the salient features of the earth’s structure into view preparatory to entering in more detail into the study of particular processes and special formations and to lay a foundation for the fuller apprehension of the successive stages of the history of the earth, which constitutes the chief purpose of geological study. It is now advisable to turn to the detailed consideration of individual processes and specific structures. The complexity of the actions involved in the history of the earth is so great that such separate consideration at the outset is helpful.