96 GENERAL REMARKS ON THE 



Little remains to be said respecting the mineral charac- 

 ters of this sandstone after the details already given in 

 other places, as it would be superfluous here to recapitu- 

 late them. It is principally distinguished from the red 

 sandstones of the centre of Scotland, by its extreme hard- 

 ness and by its crystalline texture. In this respect it often 

 equals common quartz, no distinction between the con- 

 stituent grains being visible, but the whole appearing as 

 if cemented by a general solution of silica. It is often 

 however, even in these cases, gravelly, while it occa- 

 sionally consists of large fragments, angular as well as 

 rounded, of different colours, compacted into one crystal- 

 line mass. 



One remarkable variety indeed occurs in Sutherland 

 which it is here necessary to mention, as it has not fallen 

 under review in any of the islands formerly described, 

 where the details of the composition of this rock have 

 been examined. The grains of quartz are in this case 

 cemented by a blue instead of a red clay, and the co- 

 lour of the rock varies in consequence ; passing through 

 several tints intermediate between pale grey and red, and 

 being thus occasionally of a dark brown. It is generally 

 easy to trace this blue clay to the decomposition of 

 argillaceous schist ; since, in the course of the grey beds, 

 minute fragments of that substance will be found occa- 

 sionally interspersed. In some situations these increase 

 in quantity, becoming also of larger sizes ; and thus there 

 is formed a variety of mixtures passing from a fine grey 

 sandstone to a gravelly graywacke, and ultimately to a 

 conglomerate in which large fragments of schist pre- 

 dominate. These varieties resemble in every respect but 

 colour, the graywacke and conglomerate schists that ac- 

 company the quartz rock of Scarba and Jura; the red 

 colour of the quartz, or of the felspar, which also forms a 

 part of the mixture, conferring on them a peculiar aspect. 

 In a few places it assumes characters which have been 

 supposed peculiar to certain varieties of the later red 



