latter having been sub-aerial, that is, erupted on the surface, by which the gaseous products were allowed to escape; while the former were sub-marine, having been ejected beneath the sea, or under extensive sedimentary deposits, and subjected to great pressure, by which the volatile elements were confined, and underwent new combinations. In like manner, chalk when burnt in the open air is converted into lime, the carbonic acid gas escaping; but when exposed to the same heat under pressure, it becomes granular marble. (Wond. p. 91.)

From these ancient crystalline rocks very generally underlying the sedimentary strata, and never appearing as if they had overflown from a cone or crater, the term hypogene (underlying) is employed by Mr. Lyell to designate the whole class; and they are subdivided into, Plutonic, those in which all traces of sedimentary origin are lost, as granite; and Metamorphic, those which still manifest traces of stratification, as mica-schist, &c.

The sedimentary fossiliferous rocks, are, for the convenience of study, separated into three grand

divisions.

1. The Tertiary; comprising all the deposits from the alluvial drifts to the chalk.

2. The Secondary; from the Chalk to the Old Red, or Devonian system, inclusive.

3. The Silurian; from below the Devonian to the upper part of the Cambrian system, in which all traces of organic remains disappear.

In the following arrangement the strata are

enumerated as if lying in regular order, one beneath the other; but in nature no such series has ever been observed. A few groups only occur in a serial order, and these but rarely in their original position. The strata are broken up, dislocated, and thrown into every degree of inclination, from the horizontal to the perpendicular; sometimes they are completely retroverted, the newest strata underlying those upon which they were originally deposited. The order of succession has, therefore, been ascertained by careful observation of the relative position of different members of the series in different countries ; and from an immense accumulation of facts collected by able observers from all parts of the globe. Diagrams in illustration of this table are given in Wond. plates vii. viii. ix.; and in Ly. of the different groups in the several chapters devoted to them. Dr. Buckland's Treatise contains a comprehensive illustration, drawn by Mr. Webster (Bd. Vol. II. frontispiece).

CHAPTER II.

"To discover order and intelligence, in scenes of apparent
wildness and confusion, is the pleasing task of the geological
inquirer."
DR. PARIS.

It may be necessary to remind the reader, that there are three elements of classification applicable to the stratified rocks, namely, 1st, their mineral structure, 2dly, their order of superposition, and 3dly, the nature of the organic remains which they contain; the following arrangement is in accordance with these principles.

CHRONOLOGICAL ARRANGEMENT OF THE BRITISH FORMATIONS,

COMMENCING WITH THE UPPERMOST OR NEWEST DEPOSITS.

FOSSILIFEROUS STRATA.

I. DRIFT. Comprising the superficial deposits of waterworn transported materials, and consisting of gravel, boulders, sand, clay, &c.

Subdivisions.-1. The MODERN or ALLUVIUM ; (Wond. p. 123.) characterised by the remains of man, and contemporaneous animals and plants, &c.

2. The ANCIENT or DILUVIUM; containing immense quantities of the bones of large mammalia and carnivora, of species and genera, both recent and extinct.

Observations. These beds appear to have been accumulated by a variety of causes; by land-floods, and inundations, by irruptions of the sea, and by the agency of glaciers and icebergs. They are the repository of the extinct colossal mammalia-of the mastodon, mammoth, rhinoceros, hippopotamus, elk, horse, ox, whale, &c.

II. The TERTIARY SYSTEM (Ly. I. p. 270. Wond. p. 197.). An extensive series, comprising many isolated groups of marine and lacustrine deposits, characterised by the remains of animals and vegetables of all classes, the greater portion of which are extinct. Volcanoes of great extent were in activity during this geological epoch.

Subdivisions.-1. The PLIOCENE* (more new, or recent. Ly. I. p. 286.); strata in which the shells are

* In the present state of our knowledge, this arrangement is of great utility, but it appears probable that it will require considerable modification, and perhaps must hereafter be abandoned with the progress of geological research; for it cannot be doubted, that strata in which no recent species have

for the most part of recent species, having only about ten per cent. of extinct forms.

2. The MIOCENE (less recent. Ly. I. p. 317.); containing about 20 per cent. of recent species of shells; as the Crag.

3. The EOCENE (dawn of recent, in allusion to the first appearance of recent species-Wond. p. 209. Ly. p. 337.); containing very few existing species of shells; not more than five per cent. London clay, Paris basin, &c.

Obs. The marine are often associated with fresh-water deposits, and the general characters of the Tertiary system are alternations of marine with lacustrine strata. In England the most important Tertiary deposits are those of London, the Isle of Sheppey, the western coasts of Sussex and Hampshire, the Isle of Wight, and of the eastern counties Essex, Norfolk, Suffolk, where these strata are termed Crag: (Ly. I. p. 317. Wond. p. 206.) and are subdivided into the Upper or Red Crag, and the Lower Crag.

SECONDARY FORMATIONS.

III. THE CHALK, or CRETACEOUS SYSTEM. (Wond. p. 291. Ly. I. p. 385.) A marine formation, com

yet been found, may yield them to more accurate and extended observations, and those in which but a few recent species occur with a large number of extinct forms, may have these proportions reversed.