A primer of evolution / by Edward Clodd.

  • Clodd, Edward, 1840-1930.
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: A primer of evolution / by Edward Clodd. Source: Wellcome Collection.

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    form of heat which it acquired as the molecules of which it is built up drew closer together under the action of gravitation. Although we always connect the term ‘ rock ’ with hardness, it is applied by geologists to all materials of the earth’s crust, whether drifting sand or swampy mud or granite. Rocks are divided into two classes — ujistratified and stratified. The unstratified, which are also called igneous or Plutonic (from the Greek Pluto, ruler of the dark underworld), include all rocks which, as they now exist, have been fused together by heat, or erupted from the earth’s interior by volcanic agency. The stratified, which are also called aque- ous or Neptunic (from the Roman sea-god Neptune), include all rocks which have been deposited as sedi- ment by the action of water or of the atmosphere, or which are due to the growth and decay of plants and animals. With this class are grouped the metamor- phiCy for the most part stratified rocks which have been changed into a crystalline state by the action of heat and pressure, resulting in effacement of their original character, and in the destruction of traces of any organic remains in them. Throughout the entire series of rocks the newer have been, and are being, formed out of the older, which, unless up- heaved, are always found at the bottom; but of the original crust probably no trace remains. The depth to which the unstratified rocks extend is unknown, and as they contain no organic remains they tell us nothing concerning the origin and sue-