172 HORNBLENDE SCHIST. 



in Isla and in lona, where there is a gneiss containing 

 clay-slate as an ingredient, while other portions of the 

 same heds include schistose hornblende and I have 

 also traced the steps by which the one graduates into 

 the other. On the skirts of Cruachan, where the ar- 

 gillaceous strata are penetrated by granite veins, there 

 is a perfect gradation into hornblende schist, at the 

 contact ; while the entangled fragments consist of 

 this latter substance, evidently produced by a change 

 o; the character of the former. And near Busta in 

 Shetland, the same occurs on a much more extensive 

 scale; some of the argillaceous strata being converted 

 into hornblende schist, and others into siliceous schist; 

 while, under peculiar circumstances, a species of 

 gneiss, resembling that of Isla and lona, is produced. 

 These facts require no comment; but the latter are 

 particularly important, because the whole gradation 

 between the three substances can be traced : the simple 

 argillaceous schist being first indurated to the state of 

 siliceous schist, and finally, as if by a greater degree 

 of the same influence, crystallized. Thus also we can 

 explain how the disposition in strata is united to the 

 chemical texture of the rock in question. To say 

 more on the origin of this substance, would be mere 

 repetition : especially after the recent discussion re- 

 specting it. The resemblance between actinolite and 

 hornblende is so great, that it is almost superfluous to 

 mention the change of this rock into actinolite schist. 

 And I have only finally to add, that it incidentally 

 accompanies limestone ; the calcareous bed being then 

 generally intermixed with crystals of hornblende, as 

 near Loch Laggan and in Ben y Gloe. 



I formerly noticed the probable remains of shells 

 which I have found in this rock, and am not aware that 

 it is the repository of any metallic minerals. As a 

 building stone, it deserves a regard which it has not yet 



