Mineralogy of Sky, 67 



ries putting on a great variety of aspects varying v^'ith the colour 

 of the base, the quality of the felspar which forms the crystals', 

 their magnitude, and the density of their aggregation. Blaven 

 offers a remarkable variety, in which solitary crystals of glassy fels- 

 par nearly two inches in length are sparingly disseminated through 

 the ground. The beautiful variety consisting of pure white crystals 

 in a ground of dark blue, which is found in Raasa, also occurs in 

 Glamich. But it would be fruitless to describe these varieties. 

 In the same hill it is found of an amygdaloidal texture, but, as far 

 as I have examined, the cavities contain only crystallizations of 

 €pidote similar to those occuring in the greenstone which I have 

 already mentioned. 



Such are the rocks which as far as my observations extended 

 form that mass, which, for want of a better general term, I have de- 

 signated by the name of mountain trap. It is however evident that 

 the rugged aspect and permanency of these hills cannot in any de- 

 gree be attributed to the clinkstone which enters into their compo- 

 sition. We have examples sufficient in Mull and in Arran to show 

 that hills of clinkstone always form in the progress of their decom- 

 position a feeble and smooth outline. The same indeed occurs in 

 these very hills, since the outline of Glamich, and of the other 

 mixed hills in which clinkstone forms the dark part, is equally tame 

 and rounded. From this knowledge we are led still more strongly 

 to conclude that the hard and serrated summits, and even the main 

 body of the hills which present them, are composed of some of the 

 varieties of greenstone before described, and hence to conjecture 

 that this substance forms their principal ingredient, and that the 

 clinkstone is only found on those outskirts where the passage or 

 change into syenite takes place. We have I fear no means even of 

 conjecturing the causes of the difference either in the disposition or 



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