203 



may assist a beginner to say, that, except in hard- 

 ness and in refusing to effervesce with acids, it 

 very strongly resembles in external characters 

 many of the compact and smooth varieties of 

 limestone. 



SECONDARY CLASS. 



LOWEST (RED) SANDSTONE. 



THIS is generally, but not necessarily, red of 

 various hues; being often grey, seldom white. 

 It consists, when most simple, of sand and red or 

 grey, clay, seldom very firmly united, and except 

 in a very few rare cases, never possessing that 

 general crystalline compactness which is often so 

 remarkable in the primary rock of the same 

 name. In addition to those two, almost essential, 

 ingredients, it may contain the sand of felspar, or 

 mica, or carbonat of lime, or the whole of these ; 

 as also, fragments of any rock of the primary 

 class, and of any size; when it becomes a conglo- 

 merate. As it is so important, in a geological 

 view, to distinguish it from the primary sand- 



