ERUPTIVE ROCKS.
Nothing is more difficult than to write a chronological history of the revolutions and changes to which the earth has been subjected during the ages which preceded the historic times. The phenomena which have concurred to fashion its enormous mass, and to give to it its present form and structure, are so numerous, so varied, and sometimes so nearly simultaneous in their action, that the records defy the powers of observation to separate them. The deposition of the sedimentary rocks has been subject to interruption during all ages of the world. Violent igneous eruptions have penetrated the sedimentary beds, elevating them in some places, depressing them in others, and in all cases disturbing their order of superposition, and ejecting masses of crystalline rocks from the incandescent centre to the surface. Amidst these perturbations, sometimes stretching over a vast extent of country, anything like a rigorous chronological record becomes impossible, for the phenomena are so continuous and complex that it is no longer possible to distinguish the fundamental from the accidental and secondary causes.
In order to render the subject somewhat clearer, the great facts relative to the progressive formation of the terrestrial globe are divided into epochs, during which the sedimentary rocks were formed in due order in the seas of the ancient world, the mud and sand in which were deposited day by day. Again, even where the line of demarcation is clearest between one formation and another, it must not be supposed there is any sharply defined line of separation between them. On the contrary, one system gradually merges into that which succeeds it. The rocks and fossils of the one gradually disappear, to be succeeded by those of the overlying series in the regular order of succession. The newly-made strata became the cemetery of the myriads of beings which lived and died in the bosom of the ocean. The rocks thus deposited were called Neptunian by the older geologists.
But while the seas of each epoch were thus building up, grain by grain, and bed by bed, the new formation out of the ruins of the older, other influences were at work, sometimes, to all appearance, impeding sometimes advancing, the great work. The Plutonic rocks—the igneous or eruptive rocks of modern geology, as we have seen above, were the great disturbing agents, and these disturbances occur in every age of the earth’s history. We shall have occasion to speak of these eruptive formations while describing the phenomena of the several epochs. But it is thought that the narrative will be made clearer and more instructive by grouping this class of phenomena into one chapter, which we place at the commencement, inasmuch as the constant reference to the eruptive rocks will thus be rendered more intelligible. To these are now added the section “Metamorphic Rocks,” from the fifth edition of the French work.
The rocks which issued from the centre of the earth in a state of fusion are found associated or interstratified with masses of every epoch, more especially with those of the more ancient strata. The formations which these rocks have originated possess great interest; first, because they enter into the composition of the terrestrial crust; secondly, because they have impressed on its surface, in the course of their eruption, some of the characteristics of its configuration and structure; finally, because, by their means, the metals which are the objects of human industry have been brought nearer to the surface. According to the order of their appearance, or as nearly so as can be ascertained, we shall class the eruptive rocks in two groups:—
I. The Volcanic Rocks, of comparatively recent origin, which have given rise to a succession of trachytes, basalts, and modern lavas. These, being of looser texture, are presumed to have cooled more rapidly than the Plutonic rocks, and at or near the surface.
II. The Plutonic Rocks, of older date, which are exemplified in the various kinds of granites, the syenites, the protogines, porphyries, &c. These differ from the volcanic rocks in their more compact crystalline structure, in the absence of tufa, as well as of pores and cavities; from which it is inferred that they were formed at considerable depths in the earth, and that they have cooled and crystallised slowly under great pressure.