KERRERA. GEOLOGY. 115 



atmosphere is also much more remarkable on them near 

 these junctions; since they are almost always found 

 to be rotten to a considerable depth. 



The clay slate is here, as in other situations, interspersed 

 with cubic crystals of pyrites, often of a large size. In 

 the vicinity of one of the junctions, a peculiar appearance 

 occurs, the chemical explanation of which is not very 

 obvious. The cavities formerly occupied by the crystals, 

 are occasionally filled with a very fine black powder, 

 which on analysis, was found to consist of sulphur mixed 

 with the black oxide of iron. 



The next rock to be noticed in the order of succession, 

 is placed here, not solely on theoretical views, but because 

 it is actually found in contact with the clay slate not 

 far from the ferry house. It is however very limited 

 in extent, nor does it necessarily follow the schist ; since 

 it is more common to find that rock immediately succeeded 

 by the sandstone or by the conglomerate hereafter to 

 be described. The substance in question is formed of 

 fragments of schist of different colours, cemented by a 

 base of the same materials more minutely divided. A 

 similar rock is not uncommon between the schist and 

 the ordinary sandstone conglomerate in other places. 

 It is impossible to give any further account of its position 

 or extent, as it is too limited to admit of accurate in- 

 vestigation. 



To this succeeds in order, that rock which, from its 

 peculiarity of structure and irregularity of position, is 

 the most difficult circumstance to explain in the com- 

 position of the island. It appears under the ordinary 

 characters of a fine sandstone and of a conglomerate ; 

 and, from its great extent on the adjoining mainland, in a 

 track much frequented by travellers, is so generally known 

 that, to many, an account of its characters will perhaps 

 be superfluous. It is too important however to pass by 

 unnoticed, even at the hazard of detailing that which 



