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GEOLOGY

OF

ENGLAND AND

WALES.

INTRODUCTION.

GEOLOGY in its widest interpretation is a History of the Earth. Its object is to enquire into the nature of the changes which the Earth has undergone from the beginning. The alterations which are now taking place on its surface in the continual waste of the land by rain, rivers, and sea; the dispersion and deposition of the material thus removed; the phenomena of earthquakes and volcanoes; the rising of land in one place, and its sinking in another; the distribution of animal and vegetable life, their growth and decay-all these incessant changes are but the continuation of the Earth's past history which we read in the consolidated mud, sand, and ooze with their included fossilized remains of animals and plants, and in the old volcanic rocks, which together constitute the outer coating or crust of the Earth. Thus the phenomena now produced by causes we can for the most part see and examine have arisen in past times from similar causes, and continued to do so during a period of time so vast that we cannot realise its length. The very beginning we can but dimly picture and speculate upon, and we need not enter upon this subject, as the special province of the geologist is only to deal with the Earth after

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it was in a fit state to receive and support life, when the proportion of land to water was probably much as it is now, and the climate and physical conditions, though ever varying over the same area during the different geological ages, were subject to the same laws and attended by analogous phenomena. Nowhere do we detect any evidence of abrupt physical change or universal catastrophe: on the contrary, through every epoch we can discern a Uniformity in cause not only in the inorganic, but also in the organic world, accompanied though it be by evidences of Evolution or the gradual development of higher and higher forms of life.

To go back to the very earliest history of the Earth, when it was part of a nebulous mass, would be to trespass upon the region of the Astronomer; and when we consider its latest history, we come upon questions which must be answered by the Geographer and the Archæologist.

In all such questions, however, we must not limit our ideas to what we see in our small tract of the British Islands; nor must we, in concluding that the physical forces have been the same throughout geological time, suppose that their action has been always of similar intensity to that of which we have definite proof in the present.

The changes we have to deal with are those represented by the crust of the Earth, or that part of it which alone is accessible to human observation. This crust is composed of different rocks which are arranged according to their method of formation into four classes-1. Aqueous, or Sedimentary;' 2. Sub-aërial; 3. Igneous, or Eruptive; and 4. Metamorphic. 1. Aqueous, or Sedimentary Rocks.-These include most rocks deposited under water, as Sand, Sandstone, Clay, Shale, Marl, Chalk, Limestone.

2. Sub-aërial Rocks.-These include deposits formed on land, as Peat, Coal, Æolian or Blown Sand.

1 Sometimes called Neptunian.

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