Mineralogy of Sky. 65 



cannot be distinguished, and which ought therefore to be con- 

 sidered a basalt, since no other criterion can be estabUshed between 

 the two, the variable proportions of hornblende and felspar alone 

 admitting of no better or more defined limit than this. More 

 generally in these mountains the greenstone assumes a large grain 

 and very coarse texture, and in some cases the separate substances 

 exhibit crystals of a quarter of an inch in dimension, while the 

 hollows which are found in the rocks are sometimes occupied by 

 detached crystals of hornblende. The felspar in these examples 

 is often of a greenish hue. This rock appears remarkably perma- 

 nent, showing few traces of waste or decomposition of the surface, 

 and it is of this particular variety that the rugged summit of Gars- 

 ven is composed. Among the finer grained varieties a remark- 

 able kind is found on the borders of the romantic lake Cor- 

 uisk, where it lies in detached blocks rolled down from the sur- 

 rounding mountains. It is honey-combed into large cavities, which 

 allow the hand to enter deeply within them, while at the same time 

 the surfaces are almost as fresh as if recently broken, showing none 

 of that rusty stain which attends the decomposition of greenstones 

 in general. These blocks are extremely sonorous, and, notwith- 

 standing their thickness, they ring when struck, with a sound as 

 great as, and precisely similar to, that of a thin vessel of cast Iron. 

 In other places the same rock is found studded over with large de- 

 tached protuberances resembling pedunculated fungi, or huge nails 

 driven into it. The last variety which I shall mention is found 

 in the same place, and it is the most remarkable, since it presents 

 a modification of trap hitherto undescribed. It forms a great por- 

 tion of that naked and wild surface which I have already described 

 in the general account of this spot, being disposed in enormous 

 smooth inclined beds extending, without fissure or trace pf decpflii- 



VOL. III. I 



