392 Dr. Mac Culloch on the Geology of 



Rona without pointing out to future observers the singular contor- 

 tions exhibited as well by the gneiss as by the hornblende rock 

 where they come into contact. I may also venture to suggest that 

 the rock of Blue Bay, described as a mixture of hornstone, chal- 

 cedony and quartz, appears to be a modification of gneiss, exceed- 

 ingly intersected by veins of quartz and felspar, and probably of 

 granite. It exhibits no genuine hornstone ; that part of it which 

 has at first sight this appearance, being an intimate mixture of 

 quartz and felspar. The chalcedony too is only that modification 

 of quartz which occurs in graphic granite, and which is well cha- 

 racterized by its peculiar waxy look. Exposure to the weather pro- 

 duces on its surface a glossy enamel, of an opacity and lustre still 

 more nearly resembling that of chalcedony, but which does not 

 penetrate into the stone. A similar enamel is to be observed in- 

 vesting the sandstone of Jura and of Schihallien, and I have also 

 specimens of the granite of Rockall, which exhibit the same appear- 

 ance. It seems to arise from a partial solution of the silex on the 

 surface, and has been observed in certain foreign sandstones, being 

 quoted as an instance of the solubility of silex in water. This 

 quartz occasionally assumes a high red hue, which is evidently to 

 be traced to the penetration and intimate mixture of red felspar. In 

 some places it is of a green colour, being penetrated by threads and 

 laminae of a substance bearing a resemblance to steatite rather than to 

 hornblende. It thus forms a greenish stone, probably different from 

 quartz coloured by actinolite which is called prase, from the same 

 stone penetrated by epidote which has obtained no name, or from 

 quartz tinged by green earth, which occurs in Rum and which, as 

 well as chalcedony when similarly coloured, is known by the name 

 of heliotrope. Yet, as it possesses the external characters and aspect 

 of prase, it may be safely referred to a species of which colour 



