Geological Palaeontology

GEOLOGICAL PALAEONTOLOGY The geological use of fossils as time-markers rests primarily on a basis of observation of the actual occurrence of the remains of the species of animals in the rocks. Very extensive collections have shown, for example, that individuals of the genus Productus are never found except in rocks of Upper Devonian, Carboniferous and Permian age : that the species Productus giganteus can only be found in rocks whose deposition occupied a comparatively small part of Lower Carboniferous time, whilst the family Productidae lived from the Upper Ordovician to the end of the Permian. These facts are a mere matter of observation; their accuracy depends upon a correct identification of fossils, and on the extent of the collections which have been studied. The importance of strati graphical geology for commercial purposes is so great that not only have all civilised countries been subjected to survey, but geological investigations have been conducted on an extensive scale all over the world even in such difficult regions as Spits bergen, Patagonia, central Africa and Mongolia. Thus the extent of the evidence as to the distribution in time of the larger groups of fossil animals is very great, and we are entitled to lay great weight on the order of appearance and period of existence of such groups.

Classification of Stratified Rocks.

The divisions recognised by William Smith were marked primarily by lithological differ ences. The Lower Lias in the Bath district is mostly clay, the Middle Lias contains hard bands of sandstone known as marl stone, the thin Upper Lias is predominantly limestone, the base of the Inferior Oolite is merely unconsolidated sand and so on. It was in such formations readily separable from one another by inspection that Smith collected the fossils on which he founded his generalisation that geological formations could be identified by the fossils which they contained.

Later workers extended this conception and used it for the division of the stratified rocks of the world into systems originally distinguished from one another by the character of their fauna. Thus R. I. Murchison separated off a great series of formations as the Silurian system, because of the occurrence in them of certain fossils, trilobites and graptolites for example, which do not occur or are rare and inconspicuous in later geological time.

The character of the basis on which this system was formed was made clear by the famous long-continued discussion between Murchison and Adam Sedgwick. The latter had mapped North Wales, dividing up the rocks of which it is composed into forma tions and grouping the whole in the Cambrian system. When after the establishment of the Silurian system by Murchison, the fossils collected by Sedgwick came to be examined, it was dis covered that the whole assemblage of Sedgwick's Cambrian fossils was that which characterised the Silurian system, and Murchison claimed that the term Cambrian was synonymous with Silurian. Thus the two conceivable methods of dividing stratified rocks into groups on the basis of their lithology and of their palae ontology were placed in opposition and the true nature of the differences had to be considered.

It is certain that every change in the character of sediments depends on a modification of the geography of the region in which they were being laid down, and that in those cases where successive rocks have an unconformable relationship to one another, there is an interval of time separating the two. It is now evident that the changes in the fauna which distinguish one formation from an other are to a great extent the result of the evolution which has proceeded during the time separating the deposition of the sedi ments involved. Thus the evidence from stratigraphy and from palaeontology should lead to the same conclusions. But the changes in fauna, depending as they do on evolution which pro ceeds with time, should occur whether or not the geography is so altered as to cause a modification in sedimentation. It should therefore be possible on palaeontological grounds to divide the rocks of a single geological formation into smaller periods not necessarily recognisable by lithological differences but displayed by a change in fauna. The first geologist to do so was Oppel, who divided the Lias into small periods which he called zones. Technically a zone consists of the sediments which were laid down during the period of existence of a particular species of animal. In practice it is usually found that they are most readily recognised not by the presence of a single species but by the co existence of a group of forms not one of which is necessarily present in each locality where the zone occurs.