202 



LIMESTONE (PRIMARY), 



A general, and, commonly, an active efferves- 

 cence on the application of dilute nitrous or 

 muriatic acids, is the well known test of lime- 

 stone; but the student must recollect, that the 

 very compact, or impure, varieties, do not always 

 effervesce freely unless they are powdered or 

 bruised. It may often be necessary to have re- 

 course to the geological position to distinguish the 

 primary from the secondary limestones ; but those 

 varieties which contain mica, hornblende, or 

 other minerals intermixed, may with safety be 

 referred to the primary class. 



COMPACT FELSPAR. 



This rock is simple, and of various colours, 

 green, brown red, pale grey, white, dark grey, 

 purple brown, black. The fracture is compact, 

 uniting the small splintery with the flat, or im- 

 perfecjly conchoidal. It is less hard than quartz, 

 but resists ordinary steel. It generally possesses 

 a peculiar aspect on the surface, resembling that 

 of wax, which is highly characteristic ; and, it 



