SKY. GEOLOGY. OVERLYING ROCKS. 385 



hesitation, from a limited observation, in describing 

 them as unquestionable instances of alternation. In the 

 last case which I shall enumerate, one regular bed of trap 

 may be traced for more than a mile, lying in a parallel 

 and undisturbed continuity between the secondary rocks. 

 On a sudden however it bends downwards so as to pass 

 through the strata immediately in contact, and then con- 

 tinues to hold its regular course for a space equally great, 

 with a thickness and parallelism as unaltered as before.* 

 I need make no commentary on these several facts since 

 the conclusions that may be deduced from them have long 

 been familiar to geologists. 



But I cannot terminate the account of this most extra- 

 ordinary and interesting part of Sky, without pointing out 

 that, of which the sketches already referred to will show 

 the details.f It is, that notwithstanding the entangled 

 fragments of the strata, the columnar disposition of the sur- 

 rounding trap is not affected. That disposition commences 

 to the southward of the nameless cascade already men- 

 tioned, and seems to be continued with a persistent regula- 

 rity to Loch Staffin whether these fragments are present 

 or not ; although they are actually more numerous and 

 remarkable here than in any part of the whole line. 



Another portion of the trap of the eastern shore which 

 appears worthy of description occurs at Sligachan. In 

 many places, as I have already noticed, the mixture of 

 the ordinary trap with the syenite is frequent, and it 

 appears to take place chiefly near the northern boundary of 

 the latter before this is thoroughly established. These 

 mixtures can therefore be traced along the shores of the 

 sound of Scalpa, the borders of Loch Eynort and of Loch 

 Sligachan, and from the latter point to a short distance 

 in the direction of Portree. In some cases there is a 

 gradation from the best characterized greenstone to the 

 most ordinary syenite, through various shades both of 



* Plate XVII. fig. 1. f Plate XVII. figs. 3, <5, 7, 9, 11, 12, 13, 15. 



