In accordance with this notion, they were designated metamorphic strata, and this term is by many geologists used as synonymous with stratified crystalline rocks. It was noticed that in some instances uncrystalline sediments had assumed a crystalline character in the immediate vicinity of certain erupted rocks; the effect of heat, or more probably of the heated solutions impregnating the last, having generated in the midst of the contiguous sediments crystalline mineral species. It was then possible that a formation uncrystalline in one part of its distribution should elsewhere become crystalline, or in-other words metamorphic; and it was conjectured that great areas of such rocks might be the stratigraphical equivalents of formations which are elsewhere uncrystalline sediments. In the Alps, for example, it was supposed that the gneisses and other crystalline schists were of mesozoic and even of cenozoic age, and similar rocks in other regions were declared to be palaeozoic; till at length it seemed, such was the extension of the doctrine of • rock metamorphism, that the sediments of any age might assume the characters of the primitive crystalline schists.

In fact, the crystalline schists of the Alps, the British islands, and the Appalachians have all in turn been claimed as altered strata of palaeozoic or more recent times. But these views have been controverted, and it has been shown that the crystalline strata which are now found in the Alps, superposed upon the uncrystalline fossiliferous sediments, are really ancient strata which were crystalline before the deposition of the latter, and in their normal position underlie them, but by great foldings and inversions have been brought to overlie them. In some instances in this region beds of apparently crystalline rocks are met with in which occur fossils like those of the uncrystalline sediments. These were regarded as further evidences of the metamorphic process which had proceeded so far as to develop a crystalline structure in the newer beds, without however obliterating their organic remains. But it has been shown that these pseudo-crystalline rocks are really sediments of the newer periods, made up of the ruins of the older and truly crystalline rocks. In many other cases, as in Wales and in eastern North America, it is found that the broken-up materials of the crystalline schists enter into the composition of the oldest palaeozoic schists, which are themselves uncrystalline.

While, therefore, it is clear that the crystalline schists were deposited from water, and, as will subsequently be seen, under conditions which, although chemically somewhat different from those of later times, did not prevent the development of organic life, it is now affirmed by one school of geologists that the great bodies of crystalline schists do not result from the alteration of any known series of uncrystalline strata; so that the division between the two established by Werner may still be retained as a fundamental one. This view is now sustained by Favre of Geneva, Sterrv Hunt, Gumbel, Credner, and others; but the opposite view, which maintains a wide-spread metamorphism of palaeozoic and more recent rocks, has been taught by very eminent names, and is still maintained in the principal geological text books and treatises. The partisans of the latter view, while asserting the comparatively recent origin of many crystalline schists, have always admitted the existence of an underlying or basal system of stratified crystalline rocks, which were supposed to be anterior in their formation to the appearance of life upon the earth, and from the apparent absence of fossils were called azoic rocks (signifying without life). In accordance with this nomenclature, the formations containing the fossil remains of plants and animals have been divided into palaeozoic, mesozoic, and cenozoic rocks (signifying ancient, middle, and recent life); while subsequent discoveries, indicating that life had already made its appearance in the so-called azoic period, have led to the substitution of the name eozoic (signifying the dawn of life). These four great divisions are made the basis of the accompanying tabular view of geological formations.

The subordinate divisions of Cambrian, Silurian, Devonian, etc, are of local origin, which, as will be seen, is also true of the names of most of the formations into which these in their turn are divided. In regard to the palaeozoic rocks, which have been most minutely studied in Great Britain and America, the names of the subdivisions recognized in these countries are given side by side. For the details of the mesozoic and cenozoic rocks, which have been made the subject of not less careful analysis and subdivision in Europe, the reader is referred elsewhere. A complete table of them is given on page 109 of Lyell's "Student's Elements of Geology" (1871).

It should, however, be borne in mind that all such divisions of the rocks are arbitrary and artificial. From the mode in which sediments have been deposited, and from the alternations of sea and land, it follows that there are breaks in the succession of the rocks, which arc often marked by a want of conformity in the arrangement of the successive formations. The sea retires from an uplifted continent, the strata become more or less disturbed, and perhaps in the course of ages partially broken down and swept away. When a new movement of the earth's crust brings this region once more beneath the sea, a new series of beds resting horizontally upon the older formation is deposited, and we have evidence, both from the relations of the strata and from the changes in the organic remains, of a break in the succession. Yet it is clear that elsewhere in the region occupied by the sea during this interval would be deposited sediments which fill up the interval. The process of deposition of sediments in the sea has never been interrupted, though the area of deposition has changed, and all breaks in the succession are local and accidental interruptions.