4S Dr. Mac Culloch's Sketch of the 



fall myself. This is confirmed by its texture as well as its colour, 

 both of them resembling those which are supposed to be charac- 

 teristic of primary limestones. In these respects indeed it possesses 

 an exact resemblance to the limestones which we find in various 

 parts of Scotland associated with schist, gneiss, and granite, of 

 which I have recorded examples in different places. From this, as 

 I have already hinted, we ought to receive with distrust any 

 attempt to distinguish the primary and secondary limestones by 

 internal characters, nay even by their external forms, since in this 

 respect also the limestone of Strath bears a perfect resemblance to 

 the primitive limestones already alluded to. The texture of this stone 

 is almost every where compact, with a fracture finely granular in 

 the surface, and varying between the splintery and conchoidal. It is 

 generally brittle, a character rare in the stratified limestones, and in 

 many places even breaks with the violence and cleanness of silice- 

 ous schist. In its chemical composition it is generally pure j but 

 where in contact with the syenite or the trap veins, becomes over- 

 loaded not only with silica but with magnesia and argil also. In 

 such situations it often contains veins and nodules of greenish 

 transparent serpentine, and adopts a variety of colours, a circum- 

 stance in which it also resembles most exactly those limestones 

 which in Glen Tilt are found in contact with granite, or at Bala- 

 hulish, and in Tirey and lona with mica slate and gneiss. The predo- 

 minant colour is grey, varying from nearly white through all shades 

 of dove colour to a dark blue grey, sometimes beautifully striped, 

 and mottled or veined ; in many situations it is of a pure snow 

 white, forming a perfect variety of statuary marble, which I shall 

 have occcasion to describe more particularly hereafter, and affording 

 as yet perhaps a solitary instance of this variety of limestone oc- 

 curring among secondary strata. When I say a solitary instance, I 



