370 SKY. GEOLOGY. OVEKLYING ROCKS. 



The only other circumstance which can at all be sup- 

 posed connected with the question of relative antiquity 

 between the syenite and hypersthene rock, is the occur- 

 rence of veins of the former substance traversing the 

 latter, an example of which is found in the valley of 

 Coruisk. If this vein could be traced to the mass of 

 syenite it might be held a sufficient ground of judgment, 

 but, under the present circumstances, it is incapable of 

 affording any assistance in solving the difficulty. 



Before terminating this account of the transitions of the 

 syenite, I may recall to the reader's mind the small mass 

 of porphyry similar to that of Rasay, which is found 

 on the point of Aird Bhornis opposite to that island, 

 and which, after the account of this rock formerly given, 

 it will be unnecessary further to notice. 



The basis of the syenite is a substance which has gene- 

 rally been received as compact felspar. In its softest 

 state it may be considered as a claystone, since it offers 

 no differences of character, while in a state of somewhat 

 greater induration it becomes a clinkstone, and when more 

 hardened is known by the name of compact felspar. As 

 specimens occur here in the simple state, they must be 

 considered mineralogically as examples of these different 

 substances; although in a geological sense they cannot 

 without troublesome circumlocution be described under 

 any but the general term of syenite already adopted. 

 The colour of the base varies from ochrey yellow and 

 obscure flesh colour to grey : it is often cavernous, the 

 cavities being filled with a ferruginous clay. In some 

 situations it contains crystals of felspar, either of the 

 same or of a different colour, and thus forms various 

 kinds of porphyry. The predominant variety however 

 is an aggregate of felspar and hornblende, in which the 

 hornblende generally bears a very small proportion to 



they rest on more satisfactory evidence than that produced by the 

 apparent or even real superposition of an unstratified ahove a stratified 

 rock. 



