a total amount of more than five miles; but as the transition and primary strata very much exceed this average, the aggregate of all the European stratified series may be considered to be at least ten miles.

CHAPTER IV.

Relation of Unstratified to Stratified Rocks.

I SHALL enter into no further details respecting the component members of each group of stratified rocks, than are represented by the lines of division and colours upon the section.* They are arranged under the old divisions of primary, transition, secondary, and tertiary series, more

For particular information respecting the mineral character and organic remains of the strata composing each series, I must refer to the numerous publications that have been devoted to these subjects. A most convenient summary of the contents of these publications will be found in De La Beche's Manual of Geology, and in Von Meyer's Palæologia, (Frankfurt, 1832); ample details respecting the English strata are given in Conybeare and Phillips's Geology of England and Wales. See also Bakewell's introduction to Geology, 1833; and Professor Phillips's article Geology, in the Encyclopædia Metropolitana; also Professor Phillips's Guide to Geology, 8vo. 1834; and De La Beche's Researches in Theoretical Geology, 8vo. 1834. The history of the organic remains of the tertiary period has been most ably elucidated in Lyell's Principles of Geology.

from a sense of the convenience of this long received arrangement, than from the reality of any strongly defined boundaries by which the strata, on the confines of each series, are separated from one another.

As the materials of stratified rocks are in great degree derived, directly or indirectly, from those which are unstratified,* it will be premature to enter upon the consideration of derivative strata, until we have considered briefly the history of the primitive formations. We therefore commence our inquiry at that most ancient period, when there is much evidence to render it probable that the entire materials of the globe were in a fluid state, and that the cause of this fluidity was heat. The form of the earth being that of an oblate spheroid, compressed at the poles, and enlarged at the equator, is that which a fluid mass would assume from revolution round its axis. The further fact, that the shortest diameter coincides with the existing axis

* In speaking of crystalline rocks of supposed igneous origin as unstratified, we adopt a distribution which, though not strictly accurate, has long been in general use among geologists. Ejected masses of granite, basalt, and lava have frequently horizontal partings, dividing them into beds of various extent and thickness, such as those which are most remarkable in what the Wernerians have called the Floetz trap formation, Pl. 1, section Fig. 6.; but they do not present that subdivision into successions of small beds, and still smaller laminae, which usually exist in sedimentary strata that have been deposited by the action of water.

of rotation, shows that this axis has been the same ever since the crust of the earth attained its present solid form.

Assuming that the whole materials of the globe may have once been in a fluid, or even a nebular state,* from the presence of intense heat, the passage of the first consolidated portions of this fluid, or nebulous matter, to a solid state may have been produced by the radiation of heat from its surface into space; the gradual abstraction of such heat would allow the particles of matter to approximate and crystallize; and the first result of this crystallization might have been the formation of a shell or crust, composed of oxidated metals and metalloids, constituting various rocks of the granitic series, around an incandescent nucleus, of melted matter, heavier than granite; such as forms the more weighty substance of basalt and compact lava.

It is now unnecessary to dwell on controversies which have prevailed during the last half century, respecting the origin of this large and important class of unstratified crystalline rocks,

The nebular hypothesis offers the most simple, and therefore the most probable theory, respecting the first condition of the material elements that compose our solar system. Mr. Whewell has shown how far this theory, supposing it to be established, would tend to exalt our conviction of the prior existence of some presiding Intelligence.-Bridgewater Treatises, No. III. Chap, vii.

which the common consent of nearly all modern geologists and chemists refers to the action of fire. The agency of central heat, and the admission of water to the metalloid bases of the earths and alkalies, offer two causes which, taken singly or conjointly, seem to explain the production and state of the mineral ingredients of these rocks; and to account for many of the grand mechanical movements that have affected the crust of the globe.

The gradations are innumerable, which connect the infinite varieties of granite, syenite, porphyry, greenstone, and basalt with the trachytic porphyries and lavas that are at this day ejected by volcanoes. Although there still remain some difficulties to be explained, there is little doubt that the fluid condition in which all unstratified crystalline rocks originally existed, was owing to the solvent power of heat; a power whose effect in melting the most solid materials of the earth we witness in the fusion of the hardest metals, and of the flinty materials of glass.*

The experiments of Mr. Gregory Watt on bodies cooled slowly after fusion; and of Sir James Hall, on reproducing artificial crystalline rocks, from the pounded ingredients of the same rocks highly heated under strong pressure; and the more recent experiments of Professor Mitscherlich, on the production of artificial crystals, by fusion of definite proportions of their component elements, have removed many of the objections, which were once urged against the igneous origin of crystalline rocks.

Beneath the whole series of stratified rocks that appear on the surface of the globe (see section Pl. 1), there probably exists a foundation of unstratified crystalline rocks; bearing an irregular surface, from the detritus of which the materials of stratified rocks have in great measure been derived,* amounting, as we have stated, to a thickness of many miles. This is indeed but a small depth, in comparison with the diameter of the globe; but small as it is, it affords certain evidence of a long series of changes and revolutions; affecting not only the mineral condition of the nascent surface of the earth, but attended also by important alterations in animal and vegetable life.

The detritus of the first dry lands, being drifted into the sea, and there spread out into extensive beds of mud and sand and gravel, would for ever have remained beneath the surface of the water, had not other forces been subsequently employed to raise them into dry land: these forces appear to have been the same expansive powers of heat and vapour which, having caused [the elevation of the first raised portions of the fundamental crystalline rocks,

Either directly, by the accumulation of the ingredients of disintegrated granitic rocks; or indirectly, by the repeated destruction of different classes of stratified rocks, the materials of which had, by prior operations, been derived from unstratified formations.