SKY. GEOLOGY. OVERLYING ROCKS. S73 



known, and the details of their variations are but little 

 instructive. The claystone in its several modificacions is 

 commonly simple in composition ; being of a dark lead 

 blue, which sometimes assumes a brighter hue, and occa- 

 sionally passes through various tints to a pale whitish grey 

 or ash colour. In many places it is porphyritic, the porphy- 

 ries putting on a great variety of aspects, which differ ac- 

 cording to the colour of the base, the quality of the felspar 

 that forms the crystals, their magnitude, and the density 

 of their aggregation. Blaven offers a remarkable variety, 

 in which solitary crystals of glassy felspar 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 at Rasay, 

 occurs also in Glamich. But it would be fruitless to 

 describe more of these modifications. In the same hill it is 

 found of an amygdaloidal texture, but, as far as I have 

 examined, the cavities contain only crystallizations of 

 epidote. 



The next great division of the unstratified rocks of 

 Sky comprises the trap, using that term in the sense 

 before defined. It is impossible however to draw a 

 boundary, either in a geological or topographic view, 

 between this and the syenite division already described; 

 the claystone modifications interfering with the whole 

 alike, and being equally incapable of being distinguished 

 in description in the one case as in the other ; while they 

 serve at the same time to unite the two by transitions 

 generally insensible. Under these circumstances it would 

 require a volume to detail the whole of the appearances, 

 nor would such a detail be productive of any corre- 

 spondent instruction ; for which reasons I must be content 

 with selecting such parts as present the greatest novelty, 

 or are attended with circumstances of the most general 

 interest. Those who may follow me will find a great deal 

 that is hot here described, although little that has not 

 been examined. 



