AGE OF MOLLUSKS. 



189 



275. Arrangement of the Rocks. The arrangement of 

 the rocks of this age was first thoroughly observed in a 

 region of the western part of England, which was in an- 

 cient times inhabited by a tribe called the Silures. Hence 

 this age is often called the Silurian age, and the rocks are 

 said to belong to the Silurian system, no matter m what 

 country they may be found. Local names have, after 

 this fashion, been given quite extensively by geologists 

 to different systems and formations, as you will see as 

 we proceed. The Silurian system has been very fully 

 examined in this country in the State of New York, and 

 it is presented to you in Fig. 101, in a section which was 

 drawn by Professor Hall in making his geological sur- 

 vey. The section exhibits the arrangement of the sys- 

 tem, together with the next to be noticed, the Devonian, 

 from the north side of Lake Ontario across New York 

 into Pennsylvania. I give below it the names of the dif- 

 ferent formations or groups as they have been applied 

 from the local associations of the rocks. 



Fig. 101. 



1. A. Upper rocks of the Azoic 

 age. 



9. I. Medina sandstone. 

 10. K. Clinton group. 



2. B. Potsdam sandstone, lowest 11. L Niagara group, 

 of the rocks of the Silurian age. 12. TO. Onondaga salt group. 



3. C. Calciferous sand rock. 



4. D. Black River limestone. 



5. E. Trenton limestone. 



6. F. Utica slate. 



a. Lake Ontario. 



7. G. Hudson River group. 



8. H. Gray sandstone and Oneida 

 conglomerate. 



13. N. Helderberg series. 



14. 0. Hamilton group. 



15. e. Tully limestone. 



16. P. Portage group. 



17. R. Chemung group. 



18. S. Old red sandstone. 



19. T. Conglomerate of the Car- 

 boniferous system. 



You see that the rocks all crop out here, and that, if 

 they laid horizontally, as they did when they were depos- 



