204 



stone, the position must be consulted where the 

 internal characters are not satisfactory. 



Some of the red varieties may be confounded 

 with some varieties of the superior sandstones, 

 such as the red marie of England ; and, in the 

 same manner, the white varieties may be mis- 

 taken for others belonging to the upper sand- 

 stones. As important geological considerations 

 may often depend on the solution of such doubts, 

 it becomes necessary to examine the geological 

 relations, whenever they may arise. 



SUPERIOR SANDSTONES. 



These consist of quartz sand alone, or of 

 that sand with carbonat of lime, or with clay of 

 various colours ; more rarely, with mica. The 

 far predominant varieties are white, or, at least, 

 pale. The circumstances under which they may 

 appear doubtful, have been already noticed. 



LIMESTONE. 



Effervescence, as in the case of the primary 

 limestones, distinguishes these from other rocks ; 

 but it must be recollected that the magnesian 



