The geological systems, as originally founded, were not very accurately separated from one another except locally. A comprehensive view of the characters of a system was taken, and accordingly the lines of demarcation between the same systems adopted by workers in different countries were by no means necessarily at or near the same geological horizon. As the result of more recent work, the establishment of fossil zones has been growing apace, and though many of these are seen to have only local significance, it is found as the result of experience that many of them are widely spread and occur in the same order in different localities; accordingly the remarks that have been made concerning the contemporaneity of strata apply to these zones also. After a study of this kind, a much more accurate comparison of strata is possible, and correlation of strata can be carried on to a much greater extent than when the systems were only roughly subdivided by reference to breaks, differences of lithological character, and general comparison of the faunas; accordingly whilst largely retaining the old names, the old method of classification is being partly superseded, and the included faunas alone are utilised to establish accurate correlations of the strata in various parts of the world. How far this correlation can be carried on remains to be seen, for the work though well advanced has by no means reached completion, and predictions as to the ultimate issue are useless without the experience by means of which only the work can be done. The difference between the methods of classification is well shown by an examination of the old and new divisions of the chalk. It was formerly roughly divided mainly by lithological characters into Chalk Marl, Lower Chalk without flints, Middle Chalk with few flints and Upper Chalk with many flints, but no two observers would probably agree as to where the deposit with few flints ceased and that with many commenced. The chalk is now separated on palæontological grounds into Cenomanian, Turonian, Senonian and Danian, and the superiority of the new method to the old is practically shown by the abandonment of the old classification except for very rough purposes, and the general acceptance of the new one. Many other examples might be given, but this one will suffice. In the case of some of the systems, the Carboniferous for example, the old classification founded upon lithological characters is largely extant, and it has been inferred therefore that no accurate subdivisions of the Carboniferous rocks can be made by reference to the faunas, owing to the rapidity with which the deposits were accumulated. It is by no means certain because the work has not been done that it cannot be done, and the experience obtained from a study of other strata in which subdivisions have been established by reference to the fauna would lead one to suppose that the non-establishment of subdivisions of the Carboniferous strata is due to our want of knowledge rather than to their non-existence.
The establishment of a classification on palæontological lines by no means does away with the necessity for local classifications on a lithological basis, and it has already been remarked that important results will follow from a comparison of the classifications of sediments founded on the two lines, results which have hitherto largely escaped our attention owing to the existence of a cumbrous classification attained by the application sometimes of one method, at other times of the alternative one.
SIMULATION OF STRUCTURES.
Although it is easy to give an account of the structures which are of importance to the student of the stratified rocks, actual observation of these structures is frequently attended with difficulties owing to the close imitation of one structure by another, and the past history of the science shows that erroneous conclusions have been reached again and again on account of the incorrect interpretation of structures.
Simulation of organisms has frequently been the cause of error. Inorganic substances take on the form of organisms with various degrees of closeness. The dendritic markings produced by efflorescences of oxide of manganese are familiar to all, and as the name implies, they simulate, to some extent, plant remains. More complex chemical changes have resulted in the production of rock-masses in which, not the outward form alone but, the internal structure of organisms is reproduced with more or less approach to fidelity, as the rocks which contain the supposed organisms described as Eozoon bohemicum, E. bavaricum, and, we may add, E. canadense. Mechanical changes in rocks subsequent to their formation may also cause the simulation of organisms by inorganic substances. Prof. Sollas has given reasons for considering the structure described as Oldhamia to be inorganic, and in the Carboniferous Sandstones of Little Haven, Pembrokeshire, every stage in the formation of tubular bodies resembling worm-tubes, as the result of complex folding of the strata, may be observed, whilst in other cases we find imitation of worm-tracks, as has been observed before.
It is when one inorganic structure is simulated by another that the stratigraphical geologist is most likely to be led astray, and accordingly it is worth noting some cases where this has occurred, as a warning, for it must not be supposed that the cases here noted are the only ones which are likely to occur.
It has been seen that the existence of bedding-planes is of prime importance to the geologist, and their detection is a matter of supreme moment. Under ordinary circumstances there is no great difficulty in distinguishing bedding-planes from other planes, but the importance of discovering them is often greatest when the difficulty is most pronounced. In rocks which have undergone no great amount of disturbance the planes of stratification are often marked by their regular parallelism, the separation of layers having different lithological characters by these planes, the arrangement of the longer axes of pebbles parallel to them, and the occurrence of fossils and also of rain-prints, ripple-marks and other structures produced during deposition, upon the surfaces of the strata, but none of these appearances is necessarily conclusive, especially in areas where the rocks have been subjected to orogenic movements. In regularly-jointed rocks, jointing may well be mistaken for bedding, and there is often great difficulty in discriminating between bedding and cleavage, especially when the exposures of rock are of small extent. Fossils may be dragged out along planes at an angle to the true bedding, pebbles will be compressed by cleavage so that their longer axes do not remain parallel to the bedding-planes but now lie parallel to the superinduced planes of cleavage, and a structure closely resembling 'ripple-mark' may be produced on planes other than those of original bedding, as the result of puckering. The alternation of rocks having different lithological characters may also be misleading. Intrusion of dykes along cleavage-planes, followed by decomposition of the dyke-rock causing it to resemble a sediment, and formation of mineral veins along the same planes, may give rise to an apparent succession of rocks of different lithological characters which could easily mislead an observer and cause him to mistake the cleavage-planes for planes of stratification. In rocks which have undergone great lateral pressure, the beds of different lithological character may be folded in such a way as to give very erroneous ideas of the true dip of the rock on a large scale. In [Fig. 3] the dip of the rocks in a small exposure might appear to be in the direction indicated by the unfeathered arrow, whilst the true dip of the strata as a whole, leaving the minor foldings out of account, is in the direction of the feathered arrow, at the inclination represented by the dotted line. The minor folds in a case like that represented may extend upwards for scores or even hundreds of feet, so that an error as to the direction and amount of dip may be made, even if the observer faces a cliff of considerable height.