SKY. GEOLOGY. SECONDARY STRATA. 331 



have been but cursorily mentioned, it is necessary to 

 detail them somewhat more particularly. 



It is in the first place remarkable, that with the excep- 

 tion of the very doubtful organic substances resembling 

 alcyonia, it contains no shells, although the neighbouring 

 beds into which it graduates are full of them. The 

 peculiar blueish grey colour which it acquires on weather- 

 ing, and which renders it so conspicuous at a distance, 

 has been already pointed out. This change is accom- 

 panied by a singular mode of wasting, in consequence 

 of which it becomes eroded in a cavernous manner, the 

 cavities lying on the upper surface like the cells of a 

 honeycomb, rounded interiorly, varying from six inches 

 to a foot in depth, and separated by acute narrow par- 

 titions. This appearance seems to result from the access 

 of water to its numerous fissures, which thus enlarges 

 them by a gradual solution of their sides. As it no where 

 presents seams of stratification except where it graduates 

 into the ordinary beds, and is fissured in various direc- 

 tions, it can be raised in large irregular blocks only. 

 Hence it has very naturally been considered as a primary 

 limestone. This error is confirmed by its texture as well 

 as its colour, both of them resembling those which are 

 supposed to be characteristic of primary limestones. In 

 these respects indeed it possesses an exact resemblance 

 to those which are found in various parts of Scotland asso- 

 ciated with schist, gneiss, and granite. From this we 

 ought to receive with distrust any attempt to distinguish 

 the primary and secondary limestones by internal charac- 

 ters, nay even by their external forms, since in this 

 respect also the rock of Strath bears a perfect resemblance 

 to the primary limestones already alluded to. The texture is 

 almost every where compact, the fracture being sometimes 

 finely granular in the surface, but generally varying between 

 the splintery and conchoidal. It is commonly very brittle, 

 a character rare in the stratified limestones, and in many 

 places it even breaks with the violence and cleanness of sili- 



