48 Dr. Mac Culloch's Sketch of the 



the remainder in an unconformable position to it. But it must 

 rather be considered as the indication of an internal structure, of 

 which however all marks disappear when a fracture is made, 

 a case analogous to what occurs in basalt and many other 

 rocks, of which the internal structure is so often detected by the 

 changes which they undergo on exposure to the weather. A 

 similar case, which I have observed in the argillaceous schist of 

 Isla, serves to confirm this explanation. The appearance in this 

 instance is so precisely like, that the same drawing will serve to re- 

 present both, and the nature of the cause is amply confirmed in 

 this, that while the beds of the schist are apparently divided in the 

 direction marked by the horizontal lines, they are fissile only in that 

 which is marked by the oblique ones. It hence also follows, that 

 if the fissile property of clay slate is the result of some internal 

 arrangement analogous to crystallization, we are equally entitled to 

 attribute the structure of this sandstone to the same cause. The 

 inclination of the oblique laminse upon the horizontal ones varies 

 from ten to thirty degrees ; but it is regularly in the same 

 direction, the dip, if it may be so called, being to the south. 

 This appearance is neither rare nor dubious, it is extremely well 

 marked, and predominates throughout the whole range. The 

 measurement of the parts having been mislaid, I speak from recol- 

 tion when I say that the intervals between the horizontal lines 

 vary from one foot to a foot and a half. There is no difference in 

 the quality of the two sets of laminse, both are of white sand- 

 stone, generally more or less calcareous. 



To enter more minutely into the composition of these beds, let 

 me now remark that the sandstone often acquires the aspect of 

 some of the most compact and crystalline varieties of quartz rock, 

 while in other cases it has the lax texture of an ordinary freestone. 

 In some places it is calcareous, and the calcareous matter varies so 



