Field book of common rocks and minerals

by Frederic Brewster Loomis

Igneous Rocks

Igneous rocks are those which have formed from material that has been melted, which involves temperatures around 1300° C.; or, if there is water in the original material, temperatures as low as 800° C. will suffice. Considering the increase of temperature to be a degree for every 50 feet downward, this involves the rocks having been at depths of 5 to 10 miles below the surface. While at such depths the temperature must be high enough to melt rocks, the great pressure of the overlying rocks seems to keep them solid; for we know that the center of the earth is solid, as is shown by a variety of observations, such as the rate at which earthquake waves are transmitted through the earth, the lack of tidal effects, etc. However, there is every reason to believe that if the pressure is removed from the rocks which are five to ten miles below the surface, there is heat enough at those depths to melt them. When the crust of the earth is folded, as when mountain ranges are formed, the areas under the arches or upward folds are relieved of pressure. Then those rocks, which are under the arches and are relieved, become molten. The molten magma may well up and fill the space beneath the arch where it would cool again very slowly; or, if there is fissuring during the folding, some of the molten material may be forced out through the fissures and pour out over the surface as lava. Another area in which pressures may be locally relieved is in the region of faulting, where the crust of the earth is broken into blocks, between which there are readjustments, some being tipped one way, some another, some uplifted. Here again there would be areas of relieved pressure and molten magmas would form, some of them solidifying in place, others rising to the surface.

The molten material is termed the magma, and when it reaches the surface, great quantities of water vapor and other gases escape: or these gases may even escape from magmas which do not reach the surface, rising through fissures. As these hot vapors pass through the fissures, they are cooled, and may deposit part or all of their dissolved compounds in the fissure, making veins. Lava is the magma minus the vapors. Magmas vary greatly from place to place, indicating that they are formed locally and do not come from any general interior reservoir, as has sometimes been suggested.

When the molten magmas escape to the surface, they are termed extrusive, and as they spread out in a layer this is termed a sheet. This rise and overflow may be quiet, and from time to time one outpouring may follow another making sheet after sheet. Or after one outpouring, the pressure below may cease for a time and allow the lava to solidify and make a cap or cover over the opening. Before more lava can rise, this cover must be removed. This usually happens in an explosive manner, the lava below, with the increasing pressure exerted by its expanding gases, finally exerting enough pressure, so that the cover is broken, or shattered and thrown in thousands of fragments into the air, as happened at Mt. Pelée on the Island of Martinique in 1902. The fragments thrown into the air are often termed volcanic ashes, though this is not a good word for them, for they have not been burned.

In case the molten magmas under the relieved areas do not reach the surface they are termed intrusive. Such magmas may remain in the space under a mountain fold, or be forced in fissures part way to the surface. When the magma is forced into more or less vertical cracks and there solidifies, and these are exposed by erosion, they are termed dikes. Sometimes the magmas have risen part way to the surface and then pushed their way between two horizontal layers of rock and there hardened, in which case they are termed sills, when uncovered. The Palisades along the Hudson River are the exposed edge of a sill. Again the molten magmas may well up and spread between two horizontal layers, but come faster than they can spread horizontally, and then the magma takes the form of a half sphere, and the overlying layers of rock are domed up over it. Such a mass is termed a laccolith. In all these cases the mass of igneous rock is only discovered when the overlying rocks have been eroded off. The great mass of molten magma under the arches of mountain ranges simply cools slowly into a granitic type of rock. These masses are exposed when the thousands of feet of overlying rock are eroded off. When these masses are exposed, if of but a few miles in extent, they are called stocks, but, if of many miles in length and breadth, they are batholiths, and are very characteristic of the heart of mountain ranges.

In all the above cases the exterior of the molten mass cools first, and forms a shell around the rest. The shell determines the size of the mass. As the cooling continues into the interior, it also solidifies, and as all rocks shrink on cooling, cracks develop, separating the mass into smaller pieces. There is usually no regularity about these cracks and the mass is divided into blocks from six inches to three feet in diameter. However, in some cases, especially in sills and laccoliths where the cooling is slower, the shrinkage may be marked by a regular system of cracks which bound the rock into more or less regular hexagonal columns. The Palisades and the Devil’s Tower in Wyoming (See [Plate 52]) show this structure. The Devil’s Tower is the remnant of a laccolith, all except the central core of which has been eroded away. All of the above terms have nothing to do with composition, but refer entirely to the manner of occurrence.

While the igneous rocks are classified according to their composition, the rate at which they cooled has much to do with their texture, and certain names apply to the texture. For instance when the molten lava cools very rapidly, there is no time for the formation of crystals, and the resulting rock is glassy or non-crystalline. If the cooling is slow as in large bodies, crystals have time to form and grow to considerable size as in granites. Between these all grades may occur; and one classification of igneous rocks expresses their rate of cooling, in such terms as the following.

Glassy—lavas which have cooled so quickly that they are without distinct crystallization, such as obsidian, pitchstone, etc.

Dense or felsitic—lavas which have cooled less rapidly, so that crystals have formed, but in which the crystals are too small to be identified by the unaided eye, such as felsite or basalt.

Porphyritic—magmas from which, in solidifying, one mineral has crystallized out first and the crystals have grown to considerable size, while the rest have remained small.

Granitoid—magmas which have solidified slowly, so that all the minerals have crystallized completely, and the component crystals are large enough to be recognized readily, as in granite.

Fragmental—a term applied to the fragments which have resulted from explosive eruptions of igneous rocks. These fragments may be loose or consolidated. Volcanic ashes are typical.

Porous—a term applied to the lava near the upper surface, which is filled with gas cavities, such as pumice.

Amygdoloidal—is the term applied to porous lavas, when the cavities have been filled by other minerals, such as calcite or some of the zeolites.

In determining a rock, first decide whether it is igneous, sedimentary or metamorphic. The igneous character is recognized by its being either glassy, or composed of masses of crystals irregularly arranged, there being neither layering nor bedding.

CLASSIFICATION OF IGNEOUS ROCKS

Texture	Excess of light colored minerals				Excess of dark colored minerals
Glassy	[obsidian], [perlite], [pumice], [pitchstone]				[scorias], [trachylyte], [basalt-obsidian]
	Feldspar orthoclase		Feldspar Plagioclase				No feldspar
	Mica and/or hornblende and/or augite		Mica and/or hornblende		with pyroxene		augite and/or hornblende and/or mica
	+quartz	-quartz	+quartz	-quartz	+olivine	-olivine	+olivine	-olivine
Dense	[rhyolite]	[trachite]	[dacite] (felsite)	[andesite] (felsite)	[basalt]			[augitite] or [hornblendite]
Porphyritic	[rhyolite-porphyry]	[trachite-porphyry]	[dacite-porphyry]	[andesite-porphyry]	[basalt-porphyry]			[augitite-porphyry]
Granitoid	[granite]	[syenite]	[quartz-diorite]	[diorite]	[olivine-gabbro]	[gabbro]	[peridotite]	[pyroxenite]
Fragmental	[rhyolite], [tuff] or [breccia]	[trachite], [tuff] or [breccia]	[Dacite], [tuff] or [breccia]	andesite [tuff] or [breccia]	Basalt [tuffs] and [breccias]

When it is located as igneous, turn to the key on [page 177] and decide as to which type of texture is present. If glassy, the color, luster and type of construction will place it. If the rock is crystalline, first decide whether feldspar is present, and if present, what type: then determine the dark mineral, and lastly whether quartz or olivine is present. In dense rocks the presence of quartz may be determined by trying the hardness, for none of the other constituents of igneous rocks have so great hardness. For example, if it is found that a rock is composed of orthoclase hornblende and quartz, and the texture is granitoid, it is granite: or if the rock is plagioclase feldspar and pyroxene of any sort, it is gabbro, etc.

[Granite]
[Pl. 53]

The combination of orthoclase feldspar (or microcline), quartz, and either mica, hornblende or augite is termed granite, if the texture is coarse enough so the individual minerals can be recognized with the unaided eye. The rock is light-colored because the feldspar and quartz dominate. Accessory minerals may be present such as apatite, zircon, beryl or magnetite. Varieties of granite are distinguished according to the dark mineral present. When this is muscovite, it is a muscovite-granite; when it is biotite, a biotite-granite; if it is hornblende, a hornblende-granite; etc. The size of crystals in granite varies widely. When they are as small as ¹/₁₂ of an inch in diameter, it is termed fine grained; from ¹/₁₂ to ¼ of an inch, it is medium-grained; when larger, it is coarse-grained. In some cases the crystals may be over a foot in diameter which is known as giant granite.

Originally granite was a great mass of molten magma, which has cooled very slowly, having been intruded or thrust up in great stocks or batholiths beneath overlying rocks, which acted as a blanket to prevent rapid cooling. These overlying rocks, in their turn, have been acted upon by the heat and metamorphosed. Granite is particularly likely to have been formed under mountain folds; so that, after the mountains have been more or less completely eroded away, the great masses of granite have come to the surface to mark the axes of the ranges; and even after the mountains have been wholly worn away, the granite remains to mark the sites on which they stood.

In the granite mass itself, there are often veins and dikes, which probably resulted from the shrinkage of the cooling granite, and they are filled with a different and usually coarser granite known as pegmatite. This pegmatite formed from the residual magmatic material, so that as some of the elements had already crystallized out, the granite in these dikes is of different composition. The extreme coarseness of these pegmatites seems to be due to the character of the mineralizing agents left in the dikes. In some of these pegmatites the feldspar and quartz are so intergrown, that when broken along the cleavage surface of the feldspar, the quartz appears like cuneiform characters, and this variety has been given the name graphic granite (See [Plate 53]).

When granite is exposed to weathering, the feldspar is the first mineral to be decomposed, altering eventually into carbonates, quartz and kaolin. The dark minerals are only slightly less susceptible and they break down into carbonates, iron oxides and kaolin. The original quartz remains unchanged. Of these products the carbonates, some of the iron oxide and a little of the quartz are carried away in solution. The kaolin and some of the iron oxide is in fine particles and they are carried by the water until it comes to the lakes or the sea. The quartz is left in coarser grains, which are more slowly transported, and deposited in coarser or finer sand and gravel beds.

Granites are widely used for building stone, because they can be worked readily in all directions, and have great strength and beauty. The color depends largely on the color of the feldspar, which may be white or pink, in which case the granite will be gray to pink.

Granites occur throughout New England, the Piedmont Plateau, the Lake Superior Region, the Black Hills, Rocky Mountains, Sierra Nevada, etc.

[Syenite]
[Pl. 54]

The combination of orthoclase and either mica, hornblende, or augite is syenite, the texture being coarse enough so that the individual minerals can be distinguished by the unaided eye. It differs from granite in the absence of quartz. Syenite is a light-colored rock with the feldspar predominating. Minerals like apatite, zircon, or magnetite may occur in it, as accessory minerals. The foregoing would be an ideal syenite, but usually there is some plagioclase feldspar also present. If this occurs in such quantities as to nearly equal the orthoclase feldspar, the rock is termed a monzonite; if it predominates, the rock becomes a diorite. The presence of quartz would make this rock into a granite. Such a compound rock has its type form, and when the proportions of the component minerals are changed, it grades into other types.

Like the granite, syenite is an intrusive rock, which occurs in stocks and batholiths along the axes of present or past mountain ranges. The original magma welled up under the mountain folds, where it cooled slowly, metamorphosing the adjacent rocks. Like granite it has only been exposed after a long period of erosion has removed the overlying layers of rock.

Syenites are not as abundant as granites, but they occur in the White Mountains, near Little Rock, Ark., in Custer Co., Colo., etc.

[Quartz-Diorite]

The combination of plagioclase feldspar, quartz and either mica or hornblende makes quartz-diorite, sometimes called tonalite. The above would be the typical quartz-diorite, but there is usually some orthoclase present, which if it equals the plagioclase feldspar in amount makes this into a monzonite; or if it dominates, it makes the rock a granite. Quartz-diorite is darker colored than the two preceding rocks, the dark minerals being about as abundant as the light-colored ones, such as feldspar and quartz. For this reason the weight is also somewhat greater.

Like the others this is an intrusive rock, occurring in stocks and batholiths, and indicative of great molten masses thrust up under mountain folds, and only exposed after the overlying rocks have been weathered away. It is by no means an abundant type of rock, but occurs at Belchertown, Mass., Peekskill, N. Y., in the Yellowstone Park, etc.

[Diorite]

Plagioclase feldspar with hornblende or mica, or with both, is known as diorite. It is distinguished from quartz-diorite by the absence of quartz. There is generally some augite in it, but if this should be equal to, or exceed the hornblende, the rock is then a gabbro. There may also be a small amount of orthoclase present, without taking this rock out of the diorite class, but if the orthoclase feldspar becomes dominant, then the rock is a syenite. Thus there is gradation into other groups in all directions. Apatite, magnetite, zircon, and titanite often occur in small quantities as accessory minerals. Generally the hornblende is in excess of the feldspar, so that the rock is a dark-colored one.

Diorites occur in much the same manner as granites, being in stocks, batholiths or dikes, and are often associated with granites and gabbros. They are great intruded masses, associated with mountain making, and like the preceding rocks, cooled far below the surface, and have been exposed only after great thicknesses of overlying rocks have been weathered away.

Peekskill, N. Y., the Sudbury nickel district in Canada, Mt. Davidson above the Comstock Lode in Nevada, etc., are typical localities for finding diorite.

[Olivine-Gabbro]

The combination of plagioclase feldspar with augite (or any of the pyroxenes) and olivine makes olivine-gabbro. The feldspar is usually one of those with considerable calcium in it, like labradorite; and as the dark minerals predominate, the rock is dark-colored. It is an intrusive rock, usually in dikes or stocks, where it solidified far below the surface, and was only exposed after the overlying rocks were weathered off. It is by no means an abundant type of rock, but is found in the Lake Superior Region, and near Birch Lake, Minn.

[Gabbro]
[Pl. 54]

Plagioclase feldspar with any one of the pyroxenes, most commonly augite, is gabbro. There is a wide range in the relative proportions of the two minerals making gabbro. At one extreme are rocks made entirely, or almost entirely, of plagioclase feldspar, which are known as anorthosites, and occur in parts of the higher mountains of the Adirondacks like Mt. Marcy, in several places in eastern Canada, etc. Then there are the typical gabbros where the feldspar and augite are more or less equally represented. At the other extreme come those gabbros in which the pyroxene predominates, in the most marked cases the feldspar being entirely lacking, and the rock being termed a pyroxenite. When the pyroxene of a gabbro is either enstatite or hyposthene (usually the latter) the gabbro is often called norite. Magnetite, biotite, and hornblende may occur in small quantities as accessory minerals.

Gabbro is a common intrusive rock, occurring in stocks, batholiths, and dikes, and often varies considerably in different parts of the mass. Like granite the mass solidified far below the surface, under some mountain fold, and has only been exposed as the result of weathering away the layers of overlying rock. Gabbros appear much like diorites, but are distinguished by the fact that the dark mineral is one of the pyroxenes, instead of an amphibole or a mica. They are widely distributed, being found in the White Mountains, near Peekskill, N. Y., Baltimore, Md., about Lake Superior, in Wyoming, the Rocky Mts., etc.

[Peridotite]

A rock made up of olivine and augite (or any of the pyroxenes) is peridotite. As it contains no feldspar, and both augite and olivine are dark-green to black in color, these rocks are always dark green to black in color and of considerable weight. They are usually rather coarsely crystalline. Peridotite is usually associated with gabbro, making dikes which lead from the main gabbro mass. Less frequently it occurs independently, making up an intrusive mass. Hornblende and mica may be present in small quantities, as accessory minerals.

In general these are rather rare rocks, making dikes connected with stocks or batholiths of gabbro. Peridotite is found near Baltimore, Md., in Custer Co., Colo., in Kentucky, etc.

[Pyroxenite]

This represents the extreme among coarsely crystalline igneous rocks, a whole mass made up of one mineral, and that some one of the pyroxene group. If the mineral can be exactly determined, the rock may be still more definitely named. For instance if it is all augite, then the rock would be called augitite. Like the preceding rocks, pyroxenite is an intrusive rock, usually found in dikes, which are connected with gabbro, and it represents the segregation of one mineral out of the gabbro, and its solidification at one point. Hornblende, magnetite and pyrrhotite may be present as accessory minerals. This is not a common rock, but it illustrates the fact that all possible combinations do occur, if the circumstances have warranted it. It is found near Baltimore, Md., Webster, N. C., and in Montana.

[Rhyolite]

This is a combination of orthoclase feldspar, quartz, and either hornblende, mica or augite in which the crystals are of such small size that they can not be identified with the naked eye. In composition it corresponds to granite, but it is much finer in texture. It differs from trachite by having quartz while the latter has none. This can usually be determined by trying the hardness as none of the other minerals are as hard as 7. It is much harder to distinguish it from dacite which differs only in having plagioclase feldspar in place of the orthoclase, and only the microscope will enable one to make this distinction. Where the distinction cannot be made these light-colored lavas are often called felsite.

Rhyolite is usually an extrusive lava, occurring in sheets, but sometimes it is intrusive, occurring in sills, dikes, and laccoliths. In all these cases the lava has solidified so rapidly, that the crystals are tiny, and only the general effect of a crystalline structure is distinguishable. Rhyolites may occur with porphyritic structure, in which case the presence of the larger feldspar crystals will help to distinguish whether they are orthoclase or not, making the determination easier. The color of rhyolites is green, red or gray, always a decided light shade.

Rhyolites are abundant in the western states, as in the Black Hills, the Yellowstone Park, Colorado, Nevada, California, etc.

[Trachite]

The combination of orthoclase feldspar with mica, hornblende or augite is termed trachite, if the texture is dense. It is usually an extrusive lava of light color (green, red or gray), and corresponds in composition to syenite. It can be distinguished from rhyolite by having no quartz, and so nothing to show a hardness above 5.5; but it is difficult to distinguish it from andesite, which differs only in having plagioclase feldspar in place of orthoclase. It sometimes occurs with a porphyritic structure, in which case the feldspar crystals are usually large enough to be distinguished.

Trachites are not abundant in America, but some are found in the Black Hills of South Dakota, in Custer Co., Colo., and in Montana.

[Dacite]

The union of plagioclase feldspar, quartz, and either hornblende or mica is termed dacite, if the texture is dense. It is an extrusive lava, occurring mostly in sheets and dikes. It corresponds in composition to quartz-diorite. As the texture is dense it is difficult to distinguish dacite from rhyolite, for both have quartz and differ only in the character of the feldspar, so it is quite common to use the term felsite which does not distinguish between the two, and only states that the rock is dense, light-colored and extrusive. When, as often occurs, the texture is porphyritic, and the feldspars are the large crystals, then exact determination is fairly easy.

Dacites are rather common, occurring on McClelland Peak, Nev., in the Eureka district, Nev., on Lassen’s Peak, Calif., Sepulchre Mt. in the Yellowstone Park, etc.

[Andesite]

The union of plagioclase feldspar with mica, hornblende or augite, makes andesite if the texture is dense. The lack of quartz, and so no mineral which has a hardness of over 5.5, makes it possible to distinguish andesite from dacite or rhyolite, but it is hard to distinguish this rock from trachite, which differs only on having orthoclase feldspar in place of plagioclase. When the texture is porphyritic and the feldspars are the large crystals, then it is easy to make the distinction. Andesite gets its name from being the characteristic lava of the Andes Mountains, and is the commonest of all the extruded, light-colored lavas, being the lava of hundreds of flows throughout the western United States.

The union of plagioclase feldspar and biotite is the commonest type. Plagioclase with hornblende or augite is less common, and, when they do occur, they are usually distinguished as hornblende-andesite or augite-andesite. Magnetite, apatite and zircon may be present as accessory minerals.

The lavas of Mt. Hood, Shasta, Rainier and others of the volcanic peaks of the Cascade Range, those at Eureka and Comstock in Nevada, in the Yellowstone National Park, and the porphyries of many peaks in Colorado, like the Henry Mts., etc., which are exposed laccolithic intrusions, are all andesites, as are many more.

[Basalt]

The combination of plagioclase feldspar with olivine and augite (or any other pyroxene) makes a heavy, dark-colored, black to dark-brown rock which, if its texture is dense or porphyritic, is termed basalt. This usually has more or less magnetite in it as an accessory mineral, indeed the magnetite may be so abundant as to be a component part of the rock. This magnetite makes trouble for anyone trying to use a compass on or about basalt rocks. These are extrusive or intrusive rocks and correspond in composition to gabbro.

Basalts are among the commonest of igneous rocks, and are popularly designated “trap,” much used as a road ballast on account of its toughness, which is largely due to its dense texture. The coast of New England is seamed with dikes of basalt, and through the Adirondack and White Mountains there are a host of these dikes. The crests of such mountains, as the Holyoke Range, the Tom Range, the Talcott Mts., East and West Rocks at New Haven, etc., are all basalt sheets. The Palisades, First Wachung and Second Wachung Mountains of New Jersey are sills of basalt. The Lake Superior region is crisscrossed with basalt dikes. That greatest of all lava fields the Columbia Plateau, covering over 200,000 square miles on the Snake and Columbia Rivers in Oregon, Washington and Idaho, is all basalt. So it goes all down through Nevada, New Mexico and California.

[Porphyry]
[Pl. 55]

This is a term which properly refers to texture alone, indicating a lava, which has cooled in such a manner that one mineral has crystallized out of the magma first and developed to a larger size, while the mass of the material formed tiny crystals in which the larger ones are embedded. The large crystals are technically known as phenocrysts. The surrounding mass of tiny crystals is termed the matrix. This porphyritic structure is especially characteristic of lavas which have been extruded in large masses, and of intruded lavas in such places as sills and laccoliths.

The term porphyry today has the above precise meaning. It is a much abused word, and has had all sorts of meanings. In the past it was first used to refer to lavas in general, then it came to be applied to lavas which had been erupted before Tertiary times, that is to all ancient lava sheets. This idea soon proved incorrect, lavas being of the same composition whether ancient or recent. In the West the word is often colloquially used today to designate almost every kind of igneous rock occurring in sheets or dikes, if in any way connected with ore deposits.

When the composition of a rock with porphyritic textures can be determined, the name due to the composition is coupled with that due to texture, making such terms as trachite-porphyry, basalt-porphyry, etc.

[Tuff]

Tuff, a term not to be confused with tufa on [page 215], is the name used to designate the finer fragmental ejecta of volcanic eruptions, which are also often referred to as “volcanic ash,” but the word, ash, conveys the false impression that the rock is a remnant of something burned, and is therefore not a good term. When first ejected, tuff is loose material, but it is usually soon cemented to make a more or less firm mass of rock, for which the term, tuff, is still retained. In some cases, while still loose, it is carried by streams to a distance and deposited in more or less sorted and layered beds: and the finer tuff is often carried by the winds and laid down, at a considerable distance from its source, in so called “ash beds.” In both these cases, sedimentary characteristics have been added to the tuff, and layering which is characteristic of sedimentary deposits, is present. These transported tuff beds are really sedimentary, but as there is little change in the material, they are referred to here and not again. These tuff beds are not at all uncommon in the sedimentary deposits of Tertiary age in the Rocky Mountain region. The coarser material of volcanic eruptions usually goes under the head of breccia.

[Breccia]

This term is used to describe the coarse fragmental ejecta of volcanic eruptions. It is also used, in the section under sedimentary rocks, in a broad sense to include all angular unworn fragmental material, whether of igneous or sedimentary origin. For this reason, when dealing with igneous rocks, it is usual to designate the fragments according to their composition, making such terms as trachite-breccia, rhyolite-breccia, etc.

While still loose (and also even when cemented into beds of rock), it is customary to designate the smaller fragments, from the size of a grain of wheat up to an inch or two in diameter, as lapilli; the larger fragments, from two inches up to a foot or so in diameter, as bombs; and the largest masses, often tons in weight, as volcanic blocks.

[Obsidian]
[Pl. 55]

Lavas, which have cooled so quickly that crystals have not had time to form, have a glassy appearance, and are termed obsidian. If the color is dark, due to the presence of large amounts of those elements which make dark minerals, this lava is termed basalt-obsidian. Obsidian is characterized by its glassy texture, a hardness around 6, and by breaking with a conchoidal fracture, so called because the surface is marked by a series of concentric ridges, something like the lines of growth on a shell. Obsidians vary greatly in color, but are usually red or green to black, and translucent on thin edges. While glassy, all the obsidians contain embryonic crystals, which appear like dust particles floating in the glassy matrix, or there may even be a few larger crystals present, which are often arranged in flow lines. Most all large masses of obsidian have streaks or layers of stony material in them where crystallization has set in, in a limited way.

Near the upper surface, obsidians usually have gas cavities scattered through them, and these may be small and few, or large and numerous. Indeed the cavities may be so numerous as to dominate and give the rock a frothy appearance. In this case, if the cavities are small and more or less uniform, the rock is called pumice; if they are larger it is scoria. If, as often happens when the lava is ancient and has been buried beneath other rocks, the cavities have been filled with some secondary mineral, then the lava is called an amygdoloid.

Obsidian is found in many localities, especially where there are recent volcanoes, the most famous places being the obsidian cliffs in the Yellowstone Park, those near Mono Lake in California, and many other localities in the Rocky Mountains, the Sierra Nevadas, and the Cascade Mountains.

[Pitchstone]

This is very like obsidian in appearance, but differs in that the glassy material contains from five to ten per cent of water in its composition, the most obvious effect of which is to make the luster resinous, instead of vitreous, as is characteristic of obsidian. The colors are commonly red, green or brown. Pitchstone is associated with recent volcanoes, and some fine specimens have come from Silver Cliffs, Colo., and various parts of New Mexico and Nevada.

[Perlite]
pearlstone

Perlite is a glassy lava, containing two to four per cent of water, which, on cooling, has cracked into numerous rounded masses, with a concentric structure, reminding one of the layers of an onion.

[Scoria]

While lava is cooling, there is a constant escape of gases, mostly steam, and as these rise through the molten mass they make cavities, near the upper surface, that portion on top often becoming frothy. If this solidifies quickly so that the gas cavities are preserved it is scoria. When the gas cavities are small and uniformly distributed, the rock is called pumice, and often used as a scouring agent. When the cavities are large and irregular the term scoria is generally used. Molten lavas may form various structures, according to the conditions under which they cool, dripping through cracks or from the roof of caves, which often form where the molten lava escapes from a hardened shell, and making stalactites, stalagmites, etc. The very thin lava of the Hawaiian volcanoes may even be blown by the wind into fine threads, known as “Pele’s hair.”

The presence of the gas cavities is so characteristic of the upper surface of lavas which have been extruded; that, where one is dealing with older lavas, now buried beneath other rocks, this fact helps to determine whether the mass is a sheet, rather than a sill; for, in the case of the sill, the lava was forced between layers of sedimentary rocks, and the burden of the overlying rocks did not permit the escape of steam and therefore the upper surface of sills does not have the scoriaceous structure.

[Amygdoloid]
[Pl. 56]

When the upper surface of a lava is filled with steam holes, and this lava has been buried beneath other rocks, the seeping waters slowly bring such minerals as quartz, calcite and zeolites and fill the cavities. Such a rock is known as an amygdoloid. It is often confused with porphyry; but, if examined closely, it will be seen that the outlines of the gas cavities are rounded, while the outlines of a crystal, like a phenocryst, are always angular. This will be clear if the amygdoloid on [Plate 56] is compared with the porphyry on [Plate 55].