422 SUPERIOR SANDSTONES. 



of which that change of character may be at- 

 tributed. 



Where they are most tender, the adhesion of 

 the distinct grains is produced without the ap- 

 parent intervention of another substance. The 

 most compact kinds sometimes owe their texture 

 to the cementing property of clay or of carbonat 

 of lime ; and where the compactness is still more 

 perfect, it is found that the adhesion is caused, 

 as in quartz rock, by the intermedium of a crys- 

 talline continuous cement of quartz. It is in 

 these instances that they are often difficult to dis- 

 tinguish from quartz rock except by their geolo- 

 gical position. 



Where these sandstones contain: much clay 

 or calcareous earth, or both together, and, at the 

 same time, lie in the vicinity of trap rocks, they 

 often pass into jaspers or cherts ; as is mentioned 

 in the places where these rocks are more parti- 

 cularly described. 



The minerals which enter into the formation 

 of these sandstones, are very limited ; since they 

 consist only of quartz, carbonat of lime, clay, and 

 mica. Quartz is the essential and predominant 



