near their contact with veins and dikes of volcanic rocks. These, however, are on a small scale; but a similar influence has been exerted much more powerfully in the neighbourhood of plutonic rocks under different circumstances, and perhaps in combination with other causes. The effects thereby superinduced on fossiliferous strata have sometimes extended to a distance of a quarter of a mile from the point of contact. Throughout the greater part of this space the fossiliferous beds have exchanged an earthy for a highly crystalline texture, and have lost all traces of organic remains. Thus, for example, dark limestones, replete with shells and corals, are turned into white statuary marble, and hard clays into slates called micaschist and hornblende-schist, all signs of organic bodies having been obliterated.

Although we are in a great degree ignorant of the precise nature of the influence here exerted, yet it evidently bears some analogy to that which volcanic heat and gases are capable of producing; and the action may be conveniently called plutonic, because it appears to have been developed in those regions where plutonic rocks are generated, and under similar circumstances of pressure and depth in the earth. Whether electricity or any other causes have co-operated with heat to produce this influence, may be matter of speculation, but the plutonic influence has sometimes pervaded entire mountain masses of strata. The

phenomena, therefore, being sometimes on so grand a scale, we must not consider that the strata have always assumed their crystalline or altered texture in consequence of the proximity of granite, but rather that granite itself, as well as the altered strata, have derived their crystalline texture from plutonic agency.

In accordance with this hypothesis I have proposed (see Principles of Geology), the term "Metamorphic" for the altered strata, a term derived from μera, meta, trans, and μopon, morphe, forma.

Hence there are are four great classes of rocks considered in reference to their origin, the aqueous, volcanic, plutonic, and metamorphic, all of which may be conceived to have been formed contemporaneously at every geological period, and to be now in the progress of formation. By referring to the Frontispiece, the reader will perceive what relative positions the members of these four great classes A, B, C, D, may occupy in the earth's crust, while in the course of simultaneous production. Thus, while the aqueous deposits A, which are expressed by the yellow colour, have been accumulating in successive strata at the bottom of the sea, the volcanic cone B, has been piled up during a long series of eruptions, and the other igneous rocks coloured purple have also ascended from below in a fluid state. Some of

these last have been poured forth into the sea, and there mingled with aqueous sediment. On pursuing downwards either the small dikes or large masses of volcanic rock, we find them pass gradually into plutonic formations, D, which are coloured red, and which underlie all the rest. These last again are seen to be in contact with a zone of contemporaneous metamorphic strata, C, coloured blue, which they penetrate in numerous veins.

In that part of the section which is uncoloured, a more ancient series of mineral masses are seen, belonging also to the four great divisions of rocks. The strata from a to i represent as many distinct aqueous formations, which have originated at different periods, and are each distinguished by their peculiar fossils. The mass vv is of volcanic origin, and was formed at one of those periods, namely, when the strata g were deposited. The strata m m are ancient metamorphic formations, and the rocks 1, 2, are plutonic, also ancient, but of different dates.

Now it will be shewn in the course of this volume, that portions of each of these four distinct classes of rocks have originated at many successive periods. It is not true, as was formerly supposed, that all granite, together with the crystalline or metamorphic strata, were first formed, and therefore entitled to be called "primitive," and

that the aqueous and volcanic rocks were afterwards superimposed, and should, therefore, rank as secondary in the order of time. This idea was adopted in the infancy of the science, when all formations, whether stratified or unstratified, earthy or crystalline, with or without fossils, were alike regarded as of aqueous origin. At that period it was naturally argued, that the foundation must be older than the superstructure. Granite, as being the lowest rock, must have been first "precipitated from the waters of the primeval ocean which originally invested the globe," then the crystalline, and finally the fossiliferous strata, together with other associated rocks, were deposited.

But when the doctrine of the igneous origin of granite was generally adopted, the terms primitive and primary, as embracing the plutonic and metamorphic rocks, should at once have been banished from the nomenclature of geology. For after it had been first proved that granite had originated at many different epochs, some antecedent, others subsequent to the origin of many fossiliferous strata, it was also demonstrated that strata which had once contained fossils, had become metamorphic at different periods; in other words, some of the rocks termed primary were newer than others which were called secondary. A question, therefore, has arisen, whether the

lower crystalline portions of the earth's crust, partially modified as they have been, and renewed from time to time, are newer or older, regarded as a whole, than the sedimentary and volcanic formations. Have the operations of decay and repair been most active above or below? The same question might be asked with respect to the relative antiquity of the foundations and the buildings in certain ancient cities, such as Venice or Amsterdam, which are supported on wooden pileswhether in the course of ages have the wooden props, or the buildings of brick, stone, and marble which they support, proved the most durable? Which have been renewed most frequently? for the piles, when rotten, can be removed one after the other without injury to the buildings above. In like manner the materials of the lower part of the earth's crust may pass from a solid to a fluid state, and may then again become consolidated; or sedimentary strata may assume a new and metamorphic texture, while the strata above continue unchanged, or retain characters by which their claim to high antiquity may be recognized. During such subterranean mutations, the earthquake may shatter and dislocate the incumbent crust, or the ground may rise or sink slowly and insensibly throughout wide areas *; or there

may

* See chap. 5.