434 SECONDARY LIMESTONE. 



The beds of limestone are associated in a 

 constant system of alternation with the other 

 secondary rocks, and, occasionally, even with 

 clay and with sand. As in the case of the sand- 

 stones, when, in such an association, the other 

 Substances are subordinate in quantity, they are 

 considered as subordinate in geological phrase- 

 ology, and the term formation is applied to the 

 whole ; the series being named from the lime- 

 stone. In other instances, the interposition of 

 masses of shale and sandstone, is supposed to 

 distinguish effectually between one set of cal- 

 careous strata and another ; and thus a series of 

 limestone acquires u different name, which is 

 either provincial, or, if more generally found in 

 nature, derived from some more general source. 

 Thus we have the geographical term Jura lime- 

 stone, the provincial term Lias } and the more 

 general one, mountain limestone. 



If these distinctions are sometimes arbitrary, 

 or limited and local, in others they are suffi- 

 ciently determinate and indisputable : being ac- 

 companied by certain steady and general charac- 

 ters, and, very often, by the peculiar nature of the 



