PALAEONTOLOGY AND GEOLOGY 



geology is the indispensable science by which we esti- 

 mate the time factor of evolution and arrange our 

 chain of variation. In the first place, we must remem- 

 ber that fossils can be found only in sedimentary 

 rocks and history is a blank as soon as we reach the 

 metamorphic and igneous rocks which form the core 

 of the earth a few miles below the surface. In the sec- 

 ond place, material of the sedimentary rocks is de- 

 rived from the surface disintegration of metamorphic 

 and igneous rocks exposed to the slow action of chem- 

 ical and physical forces and to the rearrangement of 

 disintegrated earlier strata of sedimentary rocks. Our 

 fossiliferous material in the earlier strata is thus 

 constantly destroyed and re-deposited and, in the pro- 

 cess, the fossils are either destroyed or are shifted to 

 the newer strata. Their mute evidence of the time 

 when they lived is either lost or correspondingly 

 shifted, it may be, to a period millions of years later; 

 so that the geologist must detect and rearrange this 

 displaced material. It is altogether improbable that 

 this has been sufficiently accomplished. We must ex- 

 pect fossils to become less and less abundant as the 

 age of the rocks increases and the mingling of fossils 

 of different epochs of time to increase as the strata 

 become more recent. 



Our estimation of time by geological methods de- 

 pends on our ability to arrange the strata of rocks in 

 the order of their actual deposition; on measuring the 

 original thickness of each stratum; on knowing the 



c 147 3 



