442 SECONDARY LIMESTONE. 



fication, and bitumen, also occur, and, in many 

 varieties, in considerable abundance. 



Where a notable proportion of quartz, or of 

 sand, is united to carbonat of lime, the texture 

 of the rock is generally very compact ; and, 

 although the particles of quartz are rarely visible, 

 such limestones give fire freely with steel. At 

 the same time, they generally eifervesce with 

 difficulty, and do not easily burn into friable lime. 

 In the vicinity of trap rocks, such limestones pass 

 into chert, as is hereafter noticed more fully under 

 that head. 



Where a large proportion of clay exists, the 

 limestones also effervesce with difficulty ; and, if 

 that substance is in great excess, refuse to burn 

 into lime. As quartz is also an ingredient in 

 these cases, it is not unfrequent for them to fuse 

 into slags in the kiln ; but when the proportion 

 of these ingredients is moderate, they produce 

 cements capable of hardening immediately on 

 the application of water, as is in other cases prac- 

 tisedj, by mixing the burnt clays with pure lime. 

 The lias strata are peculiarly noted for possessing 



