466 BUTE. GEOLOGY. 



do not exist ; and there is little doubt that a sufficiently 

 extensive examination, were that attainable, would detect 

 circumstances similar to those which, in a corresponding 

 case, have been pointed out in Sky, and of which 

 abundant examples are not wanting every where. There 

 are, indeed, some circumstances here actually present, 

 although but imperfectly marked, that tend to associate 

 this regular set of trap beds with those which have 

 been shown decidedly to interfere with the subjacent 

 rocks. These are the veins which traverse the sand- 

 stone in many places, and appear to spring from the 

 principal mass ; but no opportunity is afforded of tracing 

 them in a very unquestionable manner. 



The trap of the Garroch head sometimes presents 

 a rudely columnar aspect on its abrupt faces : more 

 generally it is amorphous. It varies in composition, 

 but is in almost all cases so deeply weathered that 

 it is extremely difficult to procure fresh specimens by 

 the hammer only. Greenstone is occasionally found in 

 it, and this has now and then the porphyritic structure. 

 But the predominant substance is that lead-blue compact 

 and uniform rock of an earthy aspect, already noticed 

 in Pladda, which has often been called greenstone, and 

 by a few, from the compactness and uniformity of its 

 texture, basalt. It is intermediate in hardness between 

 claystone and clinkstone ; the alterations of colour on 

 weathering, appearing in some cases to depend on 

 changes in the iron of its composition, while in others 

 the clay becoming disintegrated, assumes the whitish 

 pulverulent appearance that characterizes all the rocks 

 of this species as it does the compact felspars. This 

 substance, like the greenstone, is also occasionally por- 

 phyritic; the imbedded felspar being glassy, but the 

 crystals always sparingly dispersed. 



The next, and only other mass of trap, occurs at 

 Ascog : although of comparatively insignificant extent, 

 it requires particular detail; partly on its own account 



