482 Dr. Mac Culloch on Qtiartz Rock. 



from the transparent quartz grains with which it is intimately united 

 in the fresh rock. 



17. The same rock in point of external aspect, but evidently 

 formed of highly compacted and rounded grains of many different 

 colours. From the same place. 



18. Brown quartz rock, of which the fracture is so little gra- 

 nular that it almost approaches to common quartz ; semitransparent. 

 From the mountains of Mar, Angus, and elsewhere, alternating 

 with micaceous schistus. 



19. The same rock, in the same situations, of various shades 

 of red. 



20. An almost equal granular mixture of pure transparent 

 quartz, and snow-white felspar, the grains amorphous. Vmm 

 Arisaig and Balahulish, with micaceous schistus. 



21. The same, but with fragments of felspar, bearing obscure 

 marks of crystallization. From the latter place. 



22. Waxy and perfectly compact quartz, having a porphyrltic 

 aspect from imbedded fragments of felspar; accompanying the same 

 rocks* 



23. "White granular quartz rock of a moderately fine grain, con- 

 taining at the same time large angular pieces of quartz, of a diame- 

 ter from half an inch to many inches. In the same series at Balahulish. 



24. White granular rock consisting of felspar and quartz tra- 

 versed by veins of pure white granular quartz, resembling in colour 

 and texture the finest sugar. From Jura and elsewhere. 



25. Distinctly rounded grains of the purest transparent quartz, 

 imbedded in a mass of very fine grained white quartz. From the 

 Cape of Good Hope* 



26. An uniform mixture of grains of transparent white quartz and 

 opaque reddish felspar, containing; rounded pebbles and fragments 



