The Principles of Stratigraphical Geology

by J. E. Marr

Changes in the position of land and sea. Certain physicists have arrived at the conclusion that the general position of our oceans and continents was determined at a very early period in the earth's history, and that the changes which have occurred in their position since then have been comparatively insignificant. The wide extent of land over which stratified rocks are distributed at once indicates that from the point of view of the geologist the changes have been very important, and it is worth inquiring whether they are not sufficiently important to prove that the primitive oceans and continents have undergone so much alteration as to be unrecognisable. Some authorities, while recognising the great changes which have occurred in the relative position of land and sea during those periods of which geologists have direct information, suppose that the changes took place to a large degree in certain 'critical areas' bordering the more stable areas of permanent ocean on the one side and permanent land on the other.

In discussing the question of general permanence of land and ocean regions it will be convenient to commence with a study of the present land areas, and at the outset we may take into consideration the present distribution of marine sediment over different parts of the land, using the last edition of M. Jules Marcou's geological map of the world for the purpose[119]. A glimpse at this map indicates that more than half of the land areas are occupied by rocks which are as yet unknown (many of which may be marine sediments), or by crystalline schists of which the mode of origin has not yet been fully explained, so that a large part of Central Asia, the interior of Africa, and of South America may have existed as land from very early times, and the same may be said of smaller portions of Europe and North America. Actual observation of a geological map therefore indicates the possibility that about half of the land surfaces may have existed as such through very long periods, but though there is a possibility of this, the probability is not very great. The unknown regions, as remarked above, may consist to a considerable extent of marine sediments, and the existence of isolated patches of late Palæozoic and of Mesozoic strata in the heart of Central Asia, points to the submergence of much wider regions than those in which these isolated patches have been found. Again, the character of the sediments when they abut against the crystalline schists frequently proves that these sediments once extended further over the crystalline schists, and have since been removed by denudation, so that even if we assume that the crystalline schists are all of very early date, and not necessarily formed in any case from marine sediments, we cannot suppose that all the area occupied by them has existed as land for long periods of time. On the other hand, the major part of Europe and North Africa, extensive tracts in Asia, the greater part of Australia, a very large part of North America and considerable tracts of South America give proofs of having been occupied by the oceans in Palæozoic and later times.

[119] A reduced copy of this map will be found opposite the title-page of the first volume of Prof. Prestwich's Geology.

It may be answered that most of these regions containing marine sediments occur in critical areas, which have undergone a certain amount of oscillation owing to earth-movements, and that the interior parts of the great continental masses have been practically stationary. But if these lands had been land-areas through geological ages they must have been acted upon by the agents of subaerial denudation, throughout these ages, and long ago reduced to peneplains[120] unless the action of these subaerial agents was counteracted by that of elevating forces, but if these forces were sufficient to counteract the action of subaerial denudation through countless ages, they were also sufficient to raise extensive tracts of land above sea-level, and materially to alter the distribution of land and sea, and if elevation could go on to this extent, why not also depression?

[120] A term proposed by Prof. W. M. Davis for a nearly level surface of subaerial denudation, as opposed to a plain of marine denudation.

Proceeding a step further, and examining the character of the sediments as well as their geographical distribution, we find further evidence of great crust-movements. It has been urged that deep-water sediments do not occur amongst the strata found on the continents,—that there are no representatives of the abysmal deposits of recent ocean floors amongst the strata of the geological column[121], but the researches of the last two decades have brought to light foraminiferal and radiolarian deposits, pteropodal deposits, and possibly deep-sea clays, which are comparable with those in process of formation at great depths in existing oceans, and though the proofs of their deep-sea origin are not always as full as might be desired in the case of the older rocks[122], we can speak with greater certainty when we examine those of Tertiary age, and if the deep-sea accumulations of this late date can be uplifted above sea-level, this is much more likely to have occurred with those of past times. When a deposit like the radiolarian rock of Barbadoes, the deep-water character of which has been conclusively proved, can be elevated into land since Miocene or possibly Pliocene times, it is evident that the crust-movements have been sufficient to produce the most profound changes in the distribution of land and sea during the long ages which are known to us. Another argument against the occurrence of extensive changes has been derived from an examination of those islands which are spoken of as oceanic islands. Strictly speaking an oceanic island is one in which the present fauna and flora give indications of their introduction by transport across intervening sea, and no indications of the existence of forms of life which inhabited it when it was once united to a continent; it may be inferred with a considerable degree of certainty that these islands have been isolated for long periods of time. It has been stated that these oceanic islands never contain marine sediments of any considerable degree of antiquity, and that there are therefore no traces of former continents over those wide tracts of ocean which are occupied by oceanic islands. The evidence is of a negative character. The islands would be less likely to exhibit ancient sediments than continents, for being near the ocean, they would be readily submerged, and the older deposits masked by newer ones, though this need not necessarily account for the entire absence of ancient rocks amongst them. The danger of the argument lies in the fact that we do not yet know how far these old rocks really are absent, as the geology of the oceanic isles has not been fully explored from this point of view, and already several cases of the asserted presence of ancient rocks on these islands have been recorded.

[121] See Mr A. R. Wallace's Island Life.

[122] See chapter IX.

The argument derived from the present distribution of organisms is far too complex to be discussed here, and the student is recommended to read a masterly review of the evidence in Dr W. T. Blanford's Presidential Address to the Geological Society in 1890, on the question of the Permanence of Ocean Basins[123]. After reviewing the evidence furnished by a study of modern distribution he concludes that it "is far too contradictory to be received as proof of the permanence of oceans and continents."

[123] Quart. Journ. Geol. Soc., vol. XLVI., Proc., p. 59.