256 RASAY. GEOLOGY. 



head of Sky, I shall here limit myself to the description of 

 the porphyry. 



Its basis is a compact felspar, most commonly of a 

 very decided character, and rarely tending to that softer 

 variety which passes into claystone, still less into the 

 true claystone so abundant among the porphyritic rocks 

 of Arran. The colours being generally pale, the whole 

 mass has a grey aspect; and, containing much less 

 iron than the syenite of Sky, it neither moulders so 

 readily, nor have its fragments that rusty colour which 

 produces the arid appearance and red hue so charac- 

 teristic of the syenite hills of that island. The colours 

 of the basis are whitish grey passing to yellowish or 

 brownish grey, and, by other shades, to greyish blue 

 and to an obscure indigo tint. The imbedded felspar 

 crystals are white or slightly yellow ; and thus, when 

 interspersed in dark grounds, more particularly in the 

 blue, produce occasionally a very ornamental stone. In 

 decay, the crystals sometimes decompose first, leaving 

 cavities filled with ochry clay ; the whole rock assuming 

 a carious aspect. Small crystals of hornblende are 

 sometimes contained in the base, together with crystals 

 of quartz ; and these by their various proportions produce 

 specimens of great variety of aspect. 



In the different junctions which I examined, no par- 

 ticular change could be traced at the planes of contact, 

 either in the sandstone or the porphyry : if there be any, 

 it must be both slight and of rare occurrence. 



O 



I must not conclude this description of the porphyry 

 of Rasay, without pointing out the intimate connexion 

 which subsists by means of it between this island and 

 Sky ; the point of Aird Bhornis, which approaches 

 within a mile or less of Clachan Point, being formed 

 of the same rock.* 



* A sunk rock which lies in this narrow channel appears to be a 

 prolongation of this point, and to be formed of the same substance. 



