RASAY. GEOLOGY. 255 



In calling it the uppermost rock I have merely used 

 this term because the principal mass lies above the sand- 

 stone : it will be seen that it penetrates through that rock, 

 as the analogous substances trap and syenite intersect 

 the strata of Sky, and as similar rocks will hereafter be 

 shown to do in many places. An instance of this fact 

 is seen not far from the house of Clachan ; and again at 

 a lower point on the shore, where it is also found lying 

 over the sandstone ; while other similar examples occur 

 further north on the western shore. It possesses there- 

 fore, like the trap rocks, the double character of a vein 

 and an overlying mass ; or that of a mass from which 

 veins issue to penetrate and disturb the accompanying 

 regular rocks. 



This porphyry presents no regularity of disposition, 

 or at least no tendency to stratification. It has often 

 indeed a foliated structure, resembling that which oc- 

 curs in granite, but equally dissimilar to real stratifica- 

 tion. Its general resemblance to granite is often very 

 striking in other respects ; particularly in the rounded 

 surfaces and piles of prismatic bodies which it displays ; 

 while in some cases, assuming upright and rudely columnar 

 forms, it cannot on a first view be distinguished from 

 columnar trap. 



Although I have given the general term of porphyry 

 to the whole mass, on account of the predominance of 

 that substance, this rock does not exclusively occupy the 

 whole space; many portions consisting of that syenite 

 which, equally with porphyry, is found associated with 

 the ordinary trap rocks, and which forms by far the 

 predominant part of that district of Sky which most 

 resembles Rasay in character. These two rocks, formed 

 of the same materials, and only differing in the arrange- 

 ment of their parts, pass into each other by insensible 

 degrees ; but as the syenite is far inferior both in quantity 

 and variety, and will necessarily be described under the 



