SECONDARY LIMESTONE. 433 



and, in some particular instances, they undergd 

 that more complete change into chert which is 

 noticed under a subsequent head. 



These changes will be found sometimes to 

 occur in the vicinity of the trap rocks ; in others, 

 they may be observed where no such rocks are 

 present ; but as these highly interesting facts, and 

 their probable causes, are more especially matters 

 for geological discussion, the reader must here be 

 referred, for further information, to the author's 

 account of them in the work already often men- 

 tioned, where they are described as occurring in 

 the Isle of Sky and in the Isle of Man. 



Strata of limestone vary materially in thick- 

 ness, and in the parallelism of their opposed 

 planes, as those of sandstone have already been 

 shown to do ; and the accumulation of beds, 

 which forms any single series of one variety, 

 differs in the same manner in dimensions, from 

 the thickness of a few feet to masses of moun- 

 tainous bulk. In a similar way, they often form 

 very extensive tracts of country in some places ; 

 while, in others, they are limited to dimensions 

 of a few miles, or even yards. 



F F 



