132 SE1L. GEOLOGr. 



containing a few scattered crystals of glassy felspar, so 

 transparent as to permit the colour of the stone to shine 

 through them. Veins of such perfect basalt are very 

 uncommon in these islands. A much more common class 

 of veins here, consists of a grey compact felspar, weather- 

 ing first to brown and subsequently to white ; an appearance 

 described hereafter under some detail in Torsa, where 

 it is equally or more conspicuous. A few are to be seen, 

 consisting of a tolerably pure white and opake compact 

 felspar, being similar to those which occur in the neighbour- 

 ing islands. One was observed, consisting of a blueish grey 

 claystone weathering into a greenish brown, and contain- 

 ing minute black earthy spots together with a few irre- 

 gular crystals of felspar. But it would be endless to 

 enumerate all these varieties, and I shall therefore conclude 

 the description of Seil with an account of one that is 

 remarkable chiefly from its connexion with the including 

 rock.* It is found close to the town, and consists of a 

 pale grey compact felspar passing to claystone, holding 

 a course considerably erect, yet parallel to the clay 

 slate, which is here placed at a high angle. At each 

 side it is intermixed to the depth of an inch or two, with 

 distinct fragments of the schist in which it lies ; their 

 characters being in no way different from those of the 

 rock whence they have been derived. I need not stop 

 to deduce any of the more ordinary conclusions so often 

 drawn from similar appearances : it is here more important 

 to point out the confirmation which this adds to the opi- 

 nion already stated, and supported by other phenomena, 

 namely, that such rocks are truly veins even where they are 

 parallel to the strata ; and to suggest the analogy which 

 they bear to those mineral veins of which the outer surfaces 

 are formed from the fragments of the including rocks. 



It is nevertheless not unimportant here to remark, that 

 although the existence of fragments of schists among 



* Plate XXI. fur. 4. 



