THE SUCCESSION OF STRATA. 51 



CHAPTER VI. 



THE SUCCESSION <>F STRATA. 



Contemporaneous origin of loatcf -JornicJ rocks. 



The conditions under which sediments grad- 

 ually become finer, as the distance from shore and 



depth of water increase, show that all known 

 varieties of rock may be formed and deposited 

 adjacent to each other at the same time. Not 



only are the beds of peat in Irish bogs contem- 

 poraneous with the shell marls in the Loughs, but 



these are contemporaneous with the sands, clays, 



and limestones which are forming at the present 



time in our tv P L * () f mineral mat- 



ter may be represented by all the other types of 

 which layers of rock can be formed, in a siieces- 



of different localities. A geological period 



of time may be a- accurately represented by ter- 

 restrial lignite, or fresh-water sands, as by any 

 kind of marine deposit. The chalky muds dredged 

 a few hundred miles west of Ireland are accumu- 

 lated in deeper water m association with different 

 types of life, but manifestly formed contempo- 

 raneously with the shell beds of Shell Ness, the 

 muds carried out by the Thames, and the sands 

 which are spread by the tides off Yarmouth. 



In all geological ages there has been the same 

 contemporaneity of rocks of different mineral 

 character. Marine rocks must have been laid 

 down at the same time as the fresh-water sands 

 and clays of the Weald. An organic limestone 

 like the chalk formed in the open ocean, necessi- 

 tates shores where sediments were laid down. 



