Stratigraphical Geology

STRATIGRAPHICAL GEOLOGY The end and aim of stratigraphical geology is the elucidation of the history of the earth by a study of the rocks composing it. It is therefore sometimes called historical geology. For its proper comprehension it requires a knowledge of all the other branches, physical geology, petrology (including mineralogy) and palaeontol ogy. Stratigraphy may therefore be well regarded as the high est category of geology, to which all the other divisions are subsidiary, though extremely interesting in themselves. Now stratigraphy is not a mere academic science, concerned only with abstract history and dry bones. It is of enormous practical value, for on it depend many problems in mining and quarrying, en gineering, water supply, oil-finding, and a number of other sub jects essential to the well-being of the world. The technicalities of stratigraphy, though apparently of the nature of jargon and often extremely arbitrary in their application, are in reality a con venient lingua franca universally understood in the geological world and thereby a means of saving time and space in verbose explanations.

Let us now consider briefly some of the principles and methods of stratigraphical geology, in a systematic way. Two of the fundamental laws may be defined and explained as follows.

The Law of Similar Conditions.

First of all, there is the principle that certain types of deposit are correlated with cer tain physical and geographical conditions, and that this held in the past as in the present. This is a re-statement of part of the Law of Uniformity, as described in the opening of this article. In modern times this has been qualified to some extent by the reser vation that processes may have been more active in the past, differing in degree and not in kind from those now in operation. From this it follows that when we find among the older rocks certain well-defined types of deposit like those now being formed under known conditions, we are justified in drawing deductions as to the climatic and geographical conditions of the time when the rocks in question were formed.

Almost to be regarded as a corollary of the foregoing is the principle that certain types of organisms, both animals and plants, are characteristic of particular climatic and geographical con ditions. From this it follows that deposits formed at the same time, under different conditions, may contain the remains of dif ferent assemblages of plants and animals, which can be used as indicators of the nature of such conditions. This is the principle of facies, which will be discussed in detail later.

The Law of Organic Evolution.

The second great law is that organisms, regarded from the broadest biological standpoint, have developed throughout the history of the world in a certain definite order of progression from the less organized to the more organized types, from lower to higher forms of life. This of course is a mere bald statement of the general principle of evolu tion. From it follows the great generalization first stated by William Smith, that the ages of strata can be determined by means of their included fossils.

It cannot be denied that from a strictly philosophical stand point geologists are here arguing in a circle. The succession of organisms has been determined by a study of their remains em bedded in the rocks, and the relative ages of the rocks are de termined by the remains of organisms that they contain. Never theless the arguments are perfectly conclusive. This apparent paradox will disappear in the light of a little further consideration, when the necessary limitations have been introduced. The true solution of the problem lies in the combination of the two laws above stated, taking into account the actual spatial distribution of the fossil remains, which is not haphazard, but controlled by definite' laws. It is possible to a very large extent to determine the order of superposition and succession of the strata without any reference at all to their fossils. When the fossils in their turn are correlated with this succession they are found to occur in a certain definite order, and no other. Consequently, when the purely physical evidence of superposition cannot be applied, as for example to the strata of two widely separated regions, it is safe to take the fossils as a guide; this follows from the fact that when both kinds of evidence are available there is never any con tradiction between them; consequently, in the limited number of cases where only one line of evidence is available, it alone may be taken as proof.

Taking all these facts into consideration, then, it has been found possible to construct a history of the earth, at any rate from the times when conditions became comparable with what they are now. Naturally, the further back we go in time the more difficult does the construction of such a history become, for the simple reason that the events of a later date have often masked or destroyed the evidence of earlier ones. Unfortunately also it happens that certain widespread and common types of deposit are often unf os silif erous, for reasons to be discussed later.

In many writings on evolution, especially in the earlier half of the nineteenth century, much stress was laid on the "imperfection of the geological record." The chapters with this heading in Lyell's Principles and in Darwin's Origin of Species are classics in scientific literature. This argument was brought forward, and quite justifiably in the then state of knowledge, to account for gaps in the apparent order of biological evolution. At that time the geological succession of only very limited areas of the world was known, and that imperfectly. It is true that the geological suc cession in north-west Europe is unusually complete, as compared with many other parts of the world, but even there gaps are found, and certain formations are rather poorly represented by special and peculiar types of deposit. But as stratigraphical geology was extended to other parts of the world many of the gaps were filled up, and the geological record as at present known is not so incomplete as is still rather commonly believed, largely on the strength of certain of the older classical writings on evolution.

Importance of Marine Organisms.

There is also another point of great importance : in the earlier portions of geological time life seems to have been confined to the sea ; or to put it per haps more accurately and certainly in a safer form, in the earlier geological periods organisms whose remains are capable of preser vation as fossils were apparently found only in the sea. Through out the whole of geological history, in point of fact, the succession of marine animals is better known than that of terrestrial ani mals, because in the sea conditions for their preservation as fos sils are more favourable than on land. Hence stratigraphy de pends to a very large extent on marine biology, and in particular the evolutionary and palaeontological side of it has mostly been worked out on this line. Hence it follows that when any large area has been land for a considerable time we know less about the his tory of the area during that time than about the history of other parts of the earth during the same period. Consequently also, gaps in the evolutionary history of land animals during the earlier ages are specially prominent. It is only after the vertebrates ap peared that we get anything like a complete succession, and even this shows many lacunae : thus the origin of the mammals is still very obscure, as is likewise the early history of the human race.

The Geological Datum-line.

From the evolutionary char acter of the development of organisms and the poor preservation of the earlier forms, another very important consequence follows, namely, the absence of any definite base-line as a starting point in the study of the stratified formations. This difficulty is ac centuated by the high degree of alteration that most of the earlier rocks have undergone, so that the fossils, if there ever were any, have been wiped out. It is a very significant fact that the oldest known fossils are organisms of a comparatively high grade of development, highly specialized, and even in some cases showing signs of biological degeneration. It is a fair inference therefore that they must have had forerunners capable of preservation ; only by an unfortunate series of accidents all trace of these fore runners so far as yet known, has been destroyed. There is still a chance that some day they may yet be discovered in some of the less-known parts of the world, but this hope is becoming rather slender. It is quite impossible to believe that the highly organized and abundant fauna of the oldest known fossiliferous marine strata represents the absolute beginning. Such an assumption postulates a sudden special creation, and is entirely anti-evolu tionary and inconsistent with all known evidence.

Another very important consideration is that the absence of fossils in the oldest known rocks makes it very difficult to work out their relations, and especially to establish the relative ages of formations not actually seen in contact. It is always danger ous to correlate rocks in two distant areas by lithological resem blances alone, since precisely similar physical conditions may and do recur over and over again, whereas precisely similar fossil assemblages do not. Hence the stratigraphy of the oldest non f ossilif erous rocks is in a much less satisfactory state than that of the later formations. In any one, continuous area it is always possible to work out the succession of such rocks by means of the law of superposition, but it is not possible to be sure that the enormous development of these oldest rocks in Canada, for ex ample, is the time-equivalent of the very similar series in South Africa. It is only by means of fossils that it is safe to extend geological time-divisions across the sea.

Taking all these points into account, the only allowable pro cedure for the stratigrapher is to take the oldest known f ossilif er ous formation as a datum-line from which to work both upwards and downwards, just as the heights of the land and the depths of the sea are both measured from mean tide level. Above this artificial datum-line the sequence of events is clear and well known ; below it is partly guess-work.

It may be well at this point to refer to a question of nomencla ture. It is universally agreed that the oldest known fossiliferous rocks shall be called the Cambrian System. The exact meaning of the term "system" as here used is of no importance at the mo ment and will be duly explained later, as likewise will the reason for the name "Cambrian." It is only with the base-line of the system, the datum-line spoken of in the last paragraph, that we are here concerned, for a practical reason : all the rocks below this base-line are obviously older than any Cambrian rocks, hence it is convenient to speak of all of them, collectively, as the Pre Cambrian rocks. Many other names have been applied to them, some suitable, some otherwise, but this one possesses the advan tage that it is impossible to mistake its meaning.

The geological datum-line, then, is the base of the Cambrian System ; but too much importance should not be attached to it. The average school-boy usually gets the idea that before the ar rival of William the Conqueror in A.D. io66 English history was a sort of chaos : just in the same way the geologist is in danger of regarding Pre-Cambrian times as a chaotic era, whereas in both cases conditions before and after the arbitrary dates chosen were very much the same ; for though after io66 there were Norman rulers in England and after the date represented by the base of the Cambrian there were animals in the sea, the presence of neither made very much real difference. The stratified rocks of the latest Pre-Cambrian times were very like those of earliest Cam brian times, and in some places there was little physical break between. In point of fact obscure traces of organisms have been detected in Pre-Cambrian rocks, but no well-defined fauna.

Structural Relations.

In working out the geological history of any given region or of the earth as a whole we have three main lines of evidence : (a) the structural and tectonic relations of the different rock series; (b) their petrological character; (c) their fossil contents. It is by the combination of all of these that the final results are reached. Each of these categories however requires a little further elucidation, especially with regard to the relations of one line of evidence to another and the principles to be employed in combining them to form a coherent whole.

The first principle here is that a continuous and uninterrupted series of stratified rocks represents either stable conditions or a gradual sinking of the sea floor. Any discordance, that is, failure of parallelism in the strata, must indicate an interruption of some kind, usually the occurrence of earth-movement. Such a discord ance is known as an unconformity. The presence of an uncon formity as a rule implies also a certain amount of denudation in an interval between the deposition of the two sets of discordant strata. For example, to take a very simple case : if the floor of the sea is buckled up into ridges and furrows, the tops of the ridges may be planed off by tides and currents till the floor is again flat. If now deposit is resumed there must obviously be some discord ance on the site of the former ridges, while the material planed off their tops may have been deposited in the hollows between them, so that there the discordance will be less apparent. There will thus be a gap in the strata at certain points.

An

even simpler case is where a large region has been merely tilted and then planed off by denudation, newer strata being afterwards laid down in flat horizontal beds on the top : this gives rise to what is called an angular unconformity. It is obvious that in all such cases the lower series of strata must have a steeper inclination than the upper ones, unless of course a further move ment at a later date has tilted the whole back again, which often happens.

These are the conditions which must prevail if disturbance takes place in the open sea. far from a shore line. But if marine strata are raised above sea-level to form a land-surface, the effects are rather different. Let us suppose that new deposits are being formed along a coast-line with a moderate slope below sea-level. Then it is quite evident that each successive stratum of sediment laid down on the floor of the basin must extend farther towards the shore than the one below it. This relation is called overlap. The effect will obviously be more marked if the land is sinking, when marine strata may extend over what was once a land surface : when this occurs on a large scale and for a considerable distance it is called transgression.

It is impossible here to enter into detail concerning all the varieties of structure that may result from these processes under varying conditions: the point here to be made is that these phe nomena, which represent breaks or interruptions in the peaceful deposition of sediment, form convenient dividing lines in the stratigraphical succession, just as in history the end of the reign of a ruler forms a convenient end for one chapter and beginning of the next. The geological time-scale is divided arbitrarily into periods by means of these interruptions in the succession of strata; or, in other words, the major subdivisions of the stratified rocks are separated by well-marked unconformities.

Here two very important considerations arise : they may be put into simple language as follows : (a) After a disturbance caused by earth-movements, depo sition need not necessarily begin again immediately in that par ticular area, although it may be going on elsewhere. Thus there may be a long gap in the succession in any one place.

(b) Any given formation may be disturbed in any one place but not necessarily in other places. Thus a hiatus found in one locality may be bridged over by a complete series elsewhere.

These two statements really amount to the same thing, but the matter becomes clearer if they are stated separately. They have an important bearing on the subject of the imperfection of the geological record, already alluded to, and are, in fact, a re-state ment of the arguments used in dealing with that matter. The repetition is excusable, since the question is of first-rate impor tance in stratigraphical geology.

Petrological Character.

The title of this section must be taken to indicate not only the study of the actual characters and composition, both chemical and mineralogical, of the rocks, but also the evidence given by these studies as to the conditions of their formation : it is genetic rather than descriptive.

It has already been emphasized in earlier portions of this article that particular types of deposit are characteristic of par ticular sets of physical and geographical conditions. This was then specially applied to modern instances, but it is equally applicable to the older rocks, since when in course of time the incoherent superficial deposits become consolidated to form rocks, their characteristics still persist and are just as easily recogniz able. (For detailed treatment of this subject see PETROLOGY', SEDIMENTARY ROCKS.) Facies of Deposition.—Thus by a petrological study of the older rocks we are able to determine how they were formed, whether their origin was marine or terrestrial, freshwater or aeolian, and so on. Volcanic and other igneous rocks and their relations to the sediments have also to be taken into account : thus we arrive at the general conception of facies of deposition.

On broad lines it may be said that there are three chief facies of deposition, having regard to purely geographical considerations, namely, marine, freshwater and terrestrial, the term terrestrial being used of deposit under arid conditions. The glacial type of deposition is well-marked and is usually to be associated with freshwater conditions, although glaciers and icebergs do also deposit material in the sea. Each of these primary categories, however, requires further subdivision ; the types of sediment formed along the sea-shore, for instance, are distinctly different from those laid down in deep water ; the deposits of lakes and rivers also show variations; and so on. Here also, it must be mentioned, the purely physical types of sedimentation come into relationship with the variations of organic remains proper to the particular conditions, so that it is hardly possible to treat the sub ject without bringing in also its palaeontological aspect. Just as at the present time the different regions of the world each have their own fauna and flora, so in the past, owing doubtless to climatic conditions, contemporaneous life-provinces can be recognized in the strata of one and the same age. Nevertheless certain groups of organisms, such as shore-dwellers and inhabitants of deep water, are characteristic of certain particular physical conditions ; and though the particular species found in, say, the shallow water shore deposits of a particular period will not be the same all over the world, there will be a general and characteristic similarity in the groups of animals represented. Hence it appears that the question of facies of deposit cannot be properly studied without including also the fossil remains found in the rocks.

Still, as a first approximation we may consider what subdi visions are to be made, on a physical basis, in each of the three major divisions above defined. It has already been stated that the chief types of modern marine sediment are gravel, sand, mud and calcareous material. When these are consolidated they form con glomerate, sandstone, clay, mudstone, shale and limestone. Of these, conglomerates clearly indicate the close proximity of a shore line; sandstones mean shallow water deposition; clay, mudstone and shale, being formed from mud, must have been formed in deeper water; while limestones can apparently be formed in water ranging from very shallow to moderately deep, causes other than depth, such as a clear sea and warm water being favourable to their formation.

Thus we obtain the following as a useful classification :— Calcareous deposits may be found associated with any of the three. Thus some coral reefs belong to the littoral facies, shell banks are formed between tide-marks, and shells may also drift out to very considerable depths, while some molluscs live in quite deep water. The modern abyssal deposits (see OCEANOGRAPHY) are not known with certainty to be represented among the older rocks and therefore need not be considered here.

Estuarine and Delta Deposits.—A category intermediate be tween marine and freshwater deposits includes those formed in the estuaries and deltas of large rivers. The former class show most affinity to the littoral and shallow-water marine formations, since they usually contain marine fossils, but mixed up with organ isms derived from the land, notably plant-remains. The types of sediment are much the same as described above, except that calcareous deposits are generally not well developed, probably owing to the muddiness of most large rivers. In deltaic deposits terrestrial fossils usually preponderate, but marine animals some times penetrate up the rivers and become entombed. Vegetable remains are generally very abundant, owing to the growth of swamps in deltas, and sometimes form coal. Any temporary sink ing of the land, or rise of the sea may cause the intercalation of a definite marine band, as has happened in the Coal-measures at various horizons.

River Deposits.—Freshwater deposits (see ALLUVIUM) can be divided pretty clearly into those formed by rivers and in lakes. Evidently there must be a gradual transition from estuaries and delta deposits to those laid down in and along the upper parts of the course of the river; the sediment may be of any kind, except calcareous, and the fossils may obviously be a mixture of fresh water and land animals and plants. There will naturally be much variation in the kind of material deposited according to whether the river (or the particular part of it) in question flows placidly through lowlands or is a steep and rapid upland torrent. The former type lays down chiefly fine sand and silt, while the latter rolls along great boulders, stones and gravel. River deposits are therefore most like the littoral and estuarine facies from the petrographical point of view ; but the fossils are different.

Lake Deposits.—Owing to the general stillness of the water, the material laid down in lakes is finer in grain than that in river beds. There may of course be gravels and stream deltas along the shores, but in the deeper waters we usually find fine silt and mud, often calcareous, from the abundance of freshwater shells. These sometimes eventually form regular bands of rather muddy limestone. Very large lakes differ from the sea mainly in the absence of tides, and the types of sediment are much the same. The smaller lakes, anct all lakes so far as we know (except salt lakes, see below), eventually become filled up; there is usually a delta at the head, which grows steadily, and at last fills the whole basin, leading first to an alluvial flat. On top of all the other de posits there is often a bed of peat, which later forms lignite or even coal.

Desert Deposits.—The deposits belonging to the arid facies, which in point of fact is of great importance in geology, possess several distinctive features. They are generally rather coarse in texture, forming eventually breccias, conglomerates and special types of sandstone with very well-rounded grains ("millet-seed sands"). Cross-bedding and dune-structure are common, and there are no fossils; further such deposits are nearly always asso ciated with beds of rock-salt and gypsum laid down in salt-lakes, as described above, § Physical Geology. Furthermore, the strata are nearly always red, orange or yellow in colour, sometimes white, but never grey or bluish, like so many aqueous deposits. Red sandstones are by far the most characteristic feature.

Glacial Deposits.—In recent years it has been discovered that undoubted glacial deposits exist among the older rock-systems of many parts of the world. They consist of material similar to the deposits of modern glaciers, boulder clays like that of the Pleistocene ice-sheet, scratched stones, etc. Likewise they often overlie glaciated pavements with grooves and striae showing the direction of ice-movement in those ancient times. Magnificent examples have been found in South Africa and in other southern lands at several horizons, as well as in Canada, among the most ancient rocks of the mining regions of Ontario and Quebec. The glacial facies of deposit is therefore of increasing importance in stratigraphy.

In working out the geological history of any region it is of course necessary to take into account the volcanic intrusive igneous rocks that are found there. The age of these can usually be determined by considering their relations to the stratified rocks and to the disturbances that have taken place in them. It may perhaps be mentioned that igneous rocks are very often closely associated with such disturbances and frequently contempora neous with them. Earth movements and igneous activity are closely related phenomena. Thus it is logical to recognize a vol canic facies of deposit, when lavas and ashes are found inter stratified with sedimentary rocks.