54 THE DARWINIAN THEORY 



into the sea, where it is deposited in a plane over 

 the sea-bottom. The nature of the deposit will 

 depend on the source of the supply. If we ask how 

 and why the cliff is stratified, the cliff itself will tell 

 us. The stratified condition is due to deposition 

 under water. Igneous rocks are intrusive and are 

 caused by the volcanic heat of the deeper parts of 

 the earth. 



The crust of the. earth is made up of sedimentary 

 or stratified rocks deposited one above another, 

 the most recently formed being at the top. 

 Their position may be subsequently disturbed, yet 

 the general relation can usually be determined. 

 Geologists find the sequence to be much the same, 

 and to show general agreement in all parts of the 

 earth, so that the same names can be employed. 



Particular deposits may be thicker or thinner and 

 of variable nature in different localities, or absent 

 altogether. To interpret the crust of the earth 

 we must read it as a history of the earth in 

 successive chapters, like successive centuries or ages 

 in the history of mankind. The chief differences 

 are that it consists of several chapters of unequal 

 length, of which there is no means of determining 

 the absolute age or duration, separated by gaps 

 about which we have no record whatever. The 

 history of these times is revealed by fossils, " imprints 

 on the pages of time," which can be compared to the 

 descriptions and drawings in the written records of 

 man. Bones, teeth, shells, and other hard parts are 

 often found in extraordinarily perfect condition. 

 These records tell us, for example, that but a short 



