26*8 



GEOLOGY. 



megalodon, the largest species of shark, which must have 

 been 100 feet in length. The sharks of the present time 

 are large, and are justly very much feared, but they are 

 " mere pigmies," says Hitchcock, " compared with those 

 that swam in the seas which washed the shores of North 

 and South Carolina in the Eocene and Miocene periods." 

 These terrific monsters, besides being much larger than 

 the sharks of the present age, were much more numerous 

 than now. 



377. Reptiles. Some eighteen or twenty species of 

 crocodiles flourished in the Tertiary period, w r hile there 

 are only seven or eight at the present time. The croco- 

 diles whose remains are found in the London clay were 

 like those which are now living in the island of Borneo. 

 There were about seventy species of turtles and tortois- 

 es in the Tertiary period. Dana speaks of the remains 

 of one which was 20 feet in length, and whose feet must 

 have been as large as those of a rhinoceros. 



378. Mollusks. Up to the Tertiary period the mol- 

 lusks were all of species that do not exist at the present 

 time, but in this period the geologist reckons the chro- 

 nology of the strata by the relative percentage of extinct 

 and existing shelFs in them. To do this the general clas- 

 sification of Lyell ( 368) is adopted, and subdivisions, 

 more or less numerous, are made under it. In some cases 

 a stratum of rock is composed almost wholly of shells, 

 perhaps almost entirely of one kind. Sometimes there 



, is much variety in adja- 



^ 1 cent strata. As an illus- 

 tration, I will give you 

 the arrangement of a cliff 

 on the bank of the James 

 River, Va., represented 

 in Fig. 161. We have 

 here, 1. Six feet of sand 

 and clay. 2. One foot of 

 Fig- lei. reddish clav. 3. A band 



