^^^^ THE NEW EVOLUTION '"^^^ 



that extremely delicate things such as sea worms, 

 squid, or even jellyfishes, are buried in fine mud in 

 such a fashion as to prevent bacterial decay. In such 

 cases even imprints revealing the finer details of the 

 internal structure may be preserved. Such perfect 

 preservation is to be seen in some of the very earliest 

 fossils that have been discovered. 



This briefly stated is the way the remains of the 

 earlier life upon the earth have been preserved. From 

 these remains of the animals of earlier epochs in the 

 earth's history we can now form a pretty good idea of 

 the types of life that flourished at various periods in 

 the more or less distant past. 



How are we able to determine the relative age of 

 the different fossils that we find? 



Ever since rain began falling on the earth muds and 

 sands and gravels have been continuously washing 

 down into the sea. While in certain places the sedi- 

 ments now exposed may reach a thickness of about 

 two miles, in no one place has this deposition been 

 continuous. How, then, can we measure and cor- 

 relate these sediments? 



Movements of the surface of the earth subsequent 

 to the deposition of the sediments, such as those 

 involved in mountain building, often cut them 

 through or tilt them up on edge so that their cross sec- 

 tions may be studied. The erosion of streams by 

 carving great canyons through the rocks also assists 

 us in our study of them. 



By piecing together the information gathered from 

 very many regions it has proved possible to arrange 



[98] 



