Geology

Van Hise

PRE-CAMBRIAN GEOLOGY

1896

From "Principles of Pre-Cambrian North American Geology," United States Geological Survey, Sixteenth Annual Report, Pt. I, pp. 571–843, 1896.

By the term "pre-Cambrian rocks" is meant all formations which are older than those containing the Olenellus or Lower Cambrian fauna. As to the length of time represented by the pre-Cambrian, we have two points of view from which An approximate inference may be made. Accepting the nebular hypothesis, (1) we may look forward from the time when a crust first formed upon the globe, and (2) we may look backward from the broad domain of facts furnished us by the Lower Cambrian.

(1) After the first continuous solid crust of the earth formed there must have been an exceedingly long time before the conditions were such that life could exist. It is highly probable that this first outer crust was of an igneous character. After solidifying, this first shell must have thickened steadily by inward solidification. Much or all of the first crust may have been removed by subsequent erosion, but this is not true of its inward crystallization; that is, it can not be assumed that erosion has anywhere overtaken inward solidification. It is therefore to be expected that there exists in various parts of the earth the first outer crust or its inward continuation.

(2) The fauna of the Lower Cambrian is varied, complex, and abundant. It comprises all of the great classes of animals except the vertebrates, and it is by no means certain that these did not exist. One biologist says that if the differentiation of life since the Cambrian were represented by a line of a certain length the differentiation before the Cambrian would be represented by a much longer line. Another says that the amount of differentiation of life forms before the Cambrian is at least nine times as great as the amount of differentiation since the Cambrian. Believing, as we now do, that this complex and varied life was produced by slow development under a definite order set in the universe, rather than by special creations, the Cambrian life implies that the time of pre-Cambrian life was very long, although it does not necessarily imply that it is nine times as long as post-Cambrian time, for it can not be assumed that the development of life was at a uniform rate. So far as we know the laws of development, it appears to be true that higher forms differentiate more rapidly than lower forms. If this law were applied it would follow that the time of pre-Cambrian life was more than nine times as long as Cambrian and post-Cambrian time, but unknown factors may enter into the problem of the earlier phases of development, and hence we can not assume any such time ratio as nine to one. However, it seems reasonably certain that the time of pre-Cambrian life was as long as or longer, probably much longer, than all subsequent time.

If the conditions were such that life could exist, they were also such that ordinary sedimentary rocks could be deposited. Therefore it is reasonable to think that in pre-Cambrian time there were eras in which vast groups of sedimentary rocks of essentially the same character as the Paleozoic sediments were deposited. Such rocks may have since been profoundly modified.

Between the time when all the rocks forming on the earth were igneous and the time when ordinary pre-Cambrian sediments began to form, there may have been a great length of time in which the conditions were materially different from those that we now know. The waters of the ocean may have been at a high temperature, and the rocks deposited by this early ocean may have been different in character from ordinary sediments. Thus a third possible group of rocks may have formed, which bridged the time interval between the early igneous rocks and the ordinary sediments. If portions of such rocks still exist it is probable that transition phases unite them with the igneous rocks on the one hand and with ordinary sedimentary rocks on the other hand.

From our forward and backward views we then conclude that in pre-Cambrian time, at least two great classes of rocks were produced, and may, perhaps, now be found: first, those of igneous origin, representing the earliest outer crust of the earth or its inward crystallization, or both combined; second, those of sedimentary origin, deposited during the vast length of time in which life was growing from the first rudimentary form to the highly complex and varied life of the Cambrian; and, third, there may be a group of rocks intermediate in position between these two, in character and origin unlike any rocks which we now know to be forming, but connected by gradations with the later sediments, The first conclusion is a probable inference, based upon the truthfulness of the nebular hypothesis. The second conclusion is a certain one, based upon the known facts of the Cambrian life. Turning now to the field, do we find phenomena corresponding to these conclusions?

One who has worked long among the pre-Cambrian rocks in areas where the conditions are favorable for a structural study is usually impressed by the dual character of the pre-Cambrian. First, he finds a great series of gneissoid granites, gneisses, and schists, all completely crystalline, having the most intricate relations with one another, and showing the effects of repeated strong dynamic actions. These are injected by many undoubted igneous rocks of different characters and different ages. This group is always below the most ancient sedimentary rocks found. Without at present saying anything in regard to the origin of this Basement Complex, or as to the relations which obtain between it and the sedimentary rocks, it may be said that it is possible that it or some part of it corresponds either with the original crust of the earth and its inward crystallization or else with the transition rocks between these and the ordinary sediments which were deposited later. Second, he finds another great group of pre-Cambrian rocks, having all of the characteristics of ordinary sedimentary rocks. In many places these are divisible by unconformities into two or more different series. These rocks comprise conglomerates, sandstones, grits, shales, limestones, and their altered equivalents. These rocks may be reasonably regarded as occupying some part of the second great division of time, that in which life existed before the Cambrian, and as a matter of fact within these rocks we are not without evidence of the existence of life. This is demonstrated by the presence of undoubted fossils, which occur in Newfoundland, in the Grand Canyon region, in the Lake Superior country, in Great Britain, in Belgium, in Brittany, and in other places. Further, within these rocks are great beds of carbonaceous and graphitic shales, from some of which hydrocarbons may be distilled, and are rarely so rich in nongraphitic carbon as to be combustible with difficulty. That these hydrocarbons were produced by any other than organic agencies is exceedingly improbable. Also, there exist great beds of limestone and gneiss through which are disseminated particles of graphite. Finally, beds of iron carbonate and other rocks which are by many regarded as products of life are abundant.

If the position be correct that abundant life existed for eras before Cambrian time, the rarity of recognizable fossils needs explanation. This Darwin mentioned as a serious difficulty in his theory of the origin of species. Recently Brooks has given a plausible answer to this difficulty. From his biological study, and particularly from the facts of embryology, he concludes that the great stems of animal life were established when all animals were free swimming and pelagic. When the conditions became favorable, certain of the forms of the different classes availed themselves of the advantages of location at the bottom of the sea. As a consequence, these animals were limited in their habitat to length and breadth rather than length, breadth, and depth. Hence there at once arose a far keener struggle for existence than had been known before. One of the great crises in life had come. There resulted the rapid development of the hard parts, such as are found in the Cambrian rocks. Professor Brooks’s explanation^* of free swimming, pelagic animals devoid of hard parts is the most plausible one yet offered.

In a treatment of the pre-Cambrian rocks the object of which is mainly to determine the principles of stratigraphy and rules to carry them out, the question arises in what respect these rocks differ from the post-Cambrian rocks, and what are the criteria which are especially applicable to them. Why should the pre-Cambrian rocks have a treatment separate from later formations? As has been seen, the first great point in which these rocks differ from later formations is in the apparent absence of abundant fossils. Abundant life may, indeed must, have existed in some of them. Remains of this life, however, have not yet been found in sufficient quantities to serve for the purposes of correlation from district to district and region to region. Among post-Cambrian formations paleontology is the chief reliance in homotaxis and correlation. Upon paleontological data have been based nearly all inferences as to the intercontinental equivalence of strata, and even upon the same continent the equating of one set of strata with another set in a different region has been usually based upon their fossil contents. It then follows that in the pre-Cambrian there is an almost entire absence of the most important criterion upon which stratigraphical and structural work has been done among the post-Cambrian formations. We are thus driven to the use of physical data only. Being thus restricted, it becomes necessary to scrutinize them and to consider their limitations, in order that a judgment may be formed of the reliability of work done and correlations made. But the pre-Cambrian problem is still more complicated because the rocks are so old. They have been subjected to all of the constructive and destructive geological forces which have been at work during the whole of Cambrian and post-Cambrian time. Vast areas of pre-Cambrian rocks have been destroyed by the process of erosion. Other areas are hidden by later rocks. The major portion of the remnants at the surface have been profoundly modified by the processes of metamorphism.

The folding of the pre-Cambrian rocks is, upon the whole, of a more complicated character than that of the post-Cambrian, for not only have the former rocks been subjected to all of the earth movements which have affected the Cambrian and post-Cambrian sediments, but before Cambrian time they were affected by earlier movements; and if we are correct in supposing that some of these pre-Cambrian sedimentary rocks may be twice or thrice as old as those of the oldest Cambrian, the movements to which they have been subjected must have been upon the average twice or thrice as many, even if the law of uniformity applies, and it is wholly possible that the disturbing forces have increased in frequency and in power as we go backward in time, thus making the amount of folding even greater. This does not imply that there are no areas of post-Cambrian rocks which have been folded to an equal or greater degree than any pre-Cambrian rocks, but merely that in the same province the pre-Cambrian rocks are usually folded to a greater degree than later rocks; or, stated differently, the pre-Cambrian rocks have upon the average, been subjected to more periods of folding. Hence, in treating of pre-Cambrian stratigraphy we must consider the effects of this exceptional folding—what new structures have been developed, what interior alterations have occurred.

Following the same line of argument, it is plain that the jointing and faulting through which pre-Cambrian rocks have gone are, upon the whole, greater than in post-Cambrian rocks. Also, as a consequence of the folding, faulting, and jointing, brecciated or autoclastic rocks have been more extensively produced. A greater proportion of the rocks have become metamorphosed. Secondary structures have been more extensively developed in them. There have been intruded among them more numerous and abundant masses of igneous rocks; for the pre-Cambrian rocks have been subjected not only to all post-Cambrian intrusions, but to many pre-Cambrian intrusions and extrusions. It may also be that extrusive rocks were more widespread and abundant in pre-Cambrian than in post-Cambrian time. As a consequence of these various causes, producing profound effects upon the pre-Cambrian sedimentary rocks, it is inevitable that the problem of pre-Cambrian stratigraphy, even apart from the absence of fossils, is, upon the whole, a more difficult one than the problem of post-Cambrian stratigraphy; and the problem becomes increasingly difficult as we go back from the base of the Cambrian.

In the face of these difficulties, upon what criteria are we to depend in pre-Cambrian stratigraphy?

The first and most important of the criteria, as giving common horizons from which to work in comparison, is unconformity. If an unconformity is marked it must have a wide extent, for an unconformity implies an orogenic movement which raises the land above the sea; implies faulting or folding of the strata; implies truncation by means of epigene forces; and, finally, implies another orographic movement, which depresses the area below the sea. Later, evidence will be given to show that such a break can hardly be less than regional, while it may be continental or, perhaps, intercontinental in its effects. For instance, the great Appalachian revolution at the close of Paleozoic time not only caused a profound unconformity between the Paleozoic rocks and the post-Paleozoic rocks along the whole length of the Appalachian Mountains, but produced great physieal changes throughout the central and western part of the continent.

Secondly, we may use the sequence of beds of the same character as guides in equivalency; that is, if a set of beds of peculiar litho-logical character occur in like order in different districts of the same geological province, it is probable that they are parts of a once-continuous series, and if such a series of beds is separated by an unconformity from a set of beds below and another set of beds above, the supposed equivalency has an increased degree of probability. Then, with proper restrictions, the lithological character of a single bed itself may have some value in comparative Work. It is necessary that we consider carefully how an unconformity may be established, and in correlation what value may be placed upon unconformity, upon sequence of beds, and upon lithological character.

Finally, the very phenomena which make the pre-Cambrian stratigraphy a difficult problem may also give us assistance. If a lower series has been more extensively folded, or folded in a different way from a superimposed series, the question may be asked whether the two are not separated by a time interval; for if the upper series existed in the same district when the lower series was folded in the first instance, it should be folded in the same manner. Absence of folding, or a less complicated folding, indicates that the upper series was not present when the lower series was first folded. In using this criterion it must be certain that we have a case of superposition of the two series, not lateral position, for intense plications may die out rapidly transverse to the directions of the folds, and thus leave beds unaffected or little affected which were very close to those that are intensely folded. In the same way, faulting, jointing, brecciation, metamorphism, cleavage, fissility, and the relations of igneous rocks are criteria by which we may separate one series from another. For instance, in the case of igneous rocks, if a lower series is cut through and through by dikes which nowhere penetrate a superior series, the inference is that this superior series was not present at the time of the injection of the igneous rocks. Thus we see that the very causes which increase the difficulties of pre-Cambrian stratigraphy—that is, the greater amount of folding, faulting, jointing, brecciation, metamorphism, etc., to which they have been subjected—may be of assistance to us in particular cases in working out the structure.

^* The origin of the oldest fossils and the discovery of the bottom of the ocean, by W. K. Brooks. Jour. of Geol., Vol. II, pp. 455–479, 1894.