480 Dr. Mac Culloch oji Quartz Rock, 



neralization to those who may wish to assign its place in a system. 

 I have selected for description all the most prominent varieties 

 which have fallen under my notice, distinguishing their geological 

 and geographical position whenever they were known to me, and 

 marking at the same time the several gradations by which quartz 

 rock appears to pass into mica slate, into clay slate, or into gray- 

 wacke. 



1. Pure white granular rock, consisting of amorphous grains of 

 quartz, strongly agglutinated : — a perfect granular quartz. Al- 

 though the grains separately taken are transparent, the mass is 

 necessarily opaque. From Balahulish. 



2. The same rock, but containing angular fragments of white 

 felspar dispersed (although rarely) throughout it. From the same 

 place. 



3. The same rock, with extremely minute amorphous frag- 

 ments of felspar in abundance. From the same place. 



The rock whence these specimens were taken alternates with a 

 very compact micaceous schistus, and the flat surfaces which have 

 long been exposed to the weather, assume a sort of enamelled ap- 

 pearance, not much inferior to the polish given by the lapidary's 

 wheel. 



4. Pure white quartz, formed of semitransparent amorphous 

 grains, but the mass has a fracture intermediate between the gra- 

 nular and splintery. From Sky, and alternating with micaceous 

 schist. 



5. A similar rock from Balahulish, of a pink colour, but with 

 an aspect more harsh and dry, resembling common secondary sand- 

 stone. In the same situation. 



6. Pure granular quartz in the same situation, but the grains 

 of large size, and the fracture consequently very coarse. From 

 Portsoy, 



