348 SKY. GEOLOGY. SECONDARY STRATA. 



of very limited dimensions. The position of the beds is 

 so nearly horizontal that it is rarely found to vary 5, 

 except on the western shore, where it occasionally ranges 

 to 10 and 15: the dip, like that of all the stratified rocks, 

 is towards the west, the smallness of the angle in this, as 

 in all similar cases, preventing its tendency, like that of 

 the direction, from being more accurately specified. There 

 is a peculiarity in the structure of these beds so remark- 

 able, that it is necessary to point it out, and as it is dif- 

 ficult to render it intelligible in words, I have added a 

 diagram* for illustration. Each stratum seems compounded 

 of two parts, the one consisting of a single horizontal 

 lamina, and the other of an inclined series ; or, there is a 

 regular alternation of a set of inclined with one horizontal 

 lamina. These are perfectly denned, the intervals being 

 deeply channelled by the weather, and the whole having 

 the aspect of some of the carved ornaments of Saxon 

 architecture. This appearance conveys, on a first view, 

 the idea of a regular series of beds alternating with 

 each other, of which the one is horizontal, and the 

 remainder in an unconformable position to it. But it 

 must rather be considered as the indication of an in- 

 ternal structure, of which however all marks disappear 

 when a fracture is made ; a case analogous to that which 

 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 is described in the argillaceous schist 

 of Isla, serves to confirm this explanation. The appear- 

 ance in this latter instance so exactly resembles the present, 

 that the same drawing will almost serve to represent both ; 

 and the nature of the cause is amply confirmed by this 

 circumstance, that while the beds of the schist are appa- 

 rently divided in the direction marked by the horizontal 

 lines, they are fissile only in that which is marked by 



* Plate XVIII. ilg. 2. 



