The Principles of Stratigraphical Geology

by J. E. Marr

It is not for a moment suggested that the Systems have the same value, if the time taken for their accumulation be alone considered. The beds classified as Recent, for example, were probably accumulated during a lapse of time far shorter than that occupied for the deposit of some of the series or even stages of a system like the Silurian, but the recent rocks acquire a special significance from the fact that we are living in the period, and the Cainozoic rocks as a whole are capable of greater subdivision than the earlier groups, on account of the greater ease with which they can be studied, owing to the small amount of disturbance which they have usually undergone when compared with that which has affected older rocks, and the closer resemblance of their faunas and floras to those of existing times.

With reference to the groups, the writer has already commented upon the use of the terms Palæozoic, Mesozoic and Cainozoic; below the lowest Palæozoic rocks (those of the Cambrian system) lie a group of rocks which have been variously spoken of as Azoic, Eozoic, and Archæan. There is an objection to the use of any one of these words in this sense; the objection in the case of the first two is that the term is theoretical and probably incorrect, whilst the word Archæan, otherwise suitable, has also been used in a more restricted sense. In these circumstances the term Precambrian will be used when referring to any rocks which were formed below Palæozoic times, though no doubt when this obscure group of rocks is more thoroughly understood a satisfactory classification will be applied to it.

Taking the other groups into account, the lower systems of the Palæozoic group will be found to vary greatly according to the views of different writers; some make only one system, the Silurian, others two, the Cambrian and Silurian. The three systems are here adopted, not only because the one, Silurian, is too unwieldy on account of its size and requires subdivision (and the Cambrian and Silurian however defined, will be found to be of very unequal importance, whereas the three systems adopted are of fairly equal value), but especially because when the term Ordovician is used, the significance of the other terms Cambrian and Silurian is at once understood.

An attempt has been made to shew that the Devonian system is non-existent, but the result of modern research is to shew that the rocks placed in this system are worthy of the distinction, both from their importance and from the distinctness of the fauna from those of the underlying and overlying systems.

The Permo-Carboniferous system is adopted, because an important group of deposits has recently been brought to light which were not represented either in the Permian or Carboniferous system as originally defined.

Some authors have advocated the union of the Permian and Triassic systems into one system placed at the base of the Mesozoic group. This is unnecessary, and would depart from the classification originally proposed, which is to be deprecated, unless there is any strong reason for it.

The Mesozoic systems are classified according to the method generally adopted. Were a fresh classification to be proposed, a portion of the Cretaceous system might be included with the Jurassic rocks, but it is better to adhere to the old classification.

The divisions of the Cainozoic rocks are hardly systems in the sense in which the term is used in the case of the older rocks, but the reason for using these smaller subdivisions has already been mentioned. The addition of the Oligocene to the original divisions suggested by Lyell has been found useful, and the term will be used in this work.

The reasons for the adoption of the particular minor subdivisions (series and stages) in the following chapters will frequently appear when the rocks of the various systems are described, and need not be further alluded to in this place.

Although most geologists describe the stratified rocks in ascending sequence beginning with the oldest, and proceeding towards the newest, others, and notably Lyell, adopted the opposite method and commenced with an account of the newest beds. The argument generally used for the latter method is that it is easier to work from the study of the known to that of the less known, and as the faunas of the newest rocks are most like the existing faunas, the student would more readily follow a description of the rocks in the order which is opposite to that in which they were deposited.