

BUTE. GEOLOGY. 467 



and partly as it is the repository of a bed of coal. 

 I did not trace it inland beyond half a mile; the 

 covering of soil impeding this investigation ; but this 

 defect is of no moment. There is no ready mode of 

 estimating the thickness .of this mass, but it may be 

 taken at 100 feet or more. When examined on the 

 shore, it appears rather to pass through the sandstone 

 than to lie over it; but there is considerable obscurity 

 in this place, as the lateral junction of the two is 

 concealed by a cavity filled with earth. If examined 

 in the interior, it is seen decidedly lying above that 

 rock, in the stream that turns Ascog mill ; but these 

 two characters are not incompatible. It may even be 

 imagined that it has once formed a bed or mass inter- 

 mediate to the sandstone ; since the prolongation of 

 the neighbouring strata, which bear marks of waste* 

 would cover it; but these conjectures are of little 

 value. In composition, it resembles the rock of the 

 Garroch head ; frequently however presenting, when 

 fresh broken, a greenish colour with the waxy aspect 

 of serpentine : but it loses both this colour and ap- 

 pearance on drying, since, like many other rocks, 

 even of this family, it contains water when in its native 

 place, which soon quits the detached specimen. It 

 presents in some places the apparent characters of a 

 greenstone ; but, on a rigid examination, it is evident, 

 that the dark material is a dark green compact felspar, 

 and the light, as might be expected, a white one. 

 This variety, as I have elsewhere remarked, has been 

 confounded with true greenstone ; the colours and the 

 speckled aspect being taken as the criterion. It may 

 not always be very easy to distinguish these two sub- 

 stances, yet it is necessary to be accurate in the 

 description of rocks ; although, as in the present case, 

 we should be compelled to resort to a circumlocution 

 in the want of a specific term. Occasionally this rock 

 becomes a regular porphyry. 



