CHAPTEK XIX 



The stratified rocks Geological periods The age of the habitable earth 

 The geological record The succession of the great vertebrate groups. 



THE materials of which the solid crust of the earth is made up 

 are, as it is perhaps hardly necessary to point out, extremely 

 varied, but whatever may be their physical or chemical characters 

 they are all spoken of by geologists as rocks. In accordance 

 with their mode of formation these rocks may be divided into two 

 chief categories, first, those which, like granite and basalt, have 

 been formed directly by cooling of molten matter, of which the 

 earth was perhaps at one time entirely composed, and, second, 

 those which, like sandstone, shale, limestone and coal, have been* 

 formed by accumulation of detritus derived from pre-existing 

 rocks or from the remains of dead organisms. The first are 

 commonly termed igneous and the second sedimentary, the 

 latter deriving their name from the fact that they have been 

 deposited under water. Owing to the manner in which they 

 have been laid down, usually in the form of gravel, sand, mud or 

 ooze, which have subsequently become hardened and consolidated 

 by the pressure of more recent deposits, the sedimentary rocks 

 form a series of layers or strata of varying thickness superposed 

 one upon another. The stratification must at first have been 

 horizontal, but, owing to the irregular shrinkage of the earth's 

 crust and to the action of volcanic upheavals, it has usually been 

 more or less disturbed, so that the planes of bedding have been 

 tilted at various angles and frequently exhibit a high degree of 

 curvature or even contortion. Moreover, rocks which were 

 originally laid down on the bed of the sea have in many cases 

 come to form part of the dry land and even to be uplifted far above 

 sea-level in the form of mountain ranges. 



The formation of sedimentary rocks must have been going on 

 ever since the earth became cool enough to admit of the con- 

 densation of aqueous vapour into water, and it is still going on 

 in the same way at the present day. Every river is continually 



