148 TORSA. GEOLOGY. 



and on the mainland near it ; and it will be found to 

 prevail throughout the whole of that district. 



Veins of claystone porphyry, similar to those of Luing, 

 are to be seen here, and apparently free from all those 

 circumstances which might excite suspicion about their 

 true nature, or lead us to imagine that they were decom- 

 posed rocks. They maintain the same character to the 

 depth of many feet ; distances to which our hammers 

 cannot reach, but which have here been exposed by the 

 operations of quarrying. That they are really weathered 

 rocks, is however proved by the state of the great vein 

 now described. In most places, even to the depth of 

 some feet, it seems to consist of a claystone, occasionally 

 becoming porphyritic, and not to be distinguished from 

 those found in Arran, so familiar to almost every geolo- 

 gist in this country. But on cutting deeply into this 

 rock, it is found that this is in reality a condition pro- 

 duced by exposure, as it gradually diminishes and entirely 

 disappears at a certain depth ; although it does not, during 

 any part of the change, exhibit the well known characters 

 of a weathered rock. The vein, where in its natural state, 

 presents a greenish grey basis of compact felspar, inter- 

 spersed with crystals of yellowish felspar, which, from 

 the obscurity of their colour, are not very easily distin- 

 guished. In the first state of change it resembles a 

 compact indurated claystone of a yellowish grey tint, the 

 imbedded crystals becoming ochrey spots. It then gra- 

 dually acquires a more lax and arenaceous texture, the 

 ochrey spots being converted into cavities filled with a 

 brown powder ; beyond which no further tendency to 

 change or decomposition can be perceived. 



This circumstance is perhaps of more general interest 

 than it appears at first sight, although I am not willing 

 to mislead myself or others by indulging in too wide 

 a field of speculation respecting it. In all the well known 

 and ordinary cases of weathering, the mechanical de- 

 struction of the rock follows soon after its decomposition ; 



