TORSA. GEOLOGY. 145 



islands present. The direction and dip of the strata 

 of Torsa, correspond exactly with those of Luing and 

 Seil ; and the prolongations of the beds in the two latter, 

 point out, as they pass through Torsa, not only the 

 boundaries, but the nature of the rocks of which it is 

 composed. 



The lowermost strata of the island form the western 

 subsidiary elevation, and are so accurately prolonged 

 upon the line of the grey ridge common to Seil and 

 Luing, between which this part of Torsa lies, as to leave 

 no doubt respecting their identity. This ridge is here 

 also marked by the prismatic fracture, and by the same 

 character of nakedness amid the surrounding verdure 

 that covers the clay slate. Hence it is visible from a 

 distance ; while the conjectures thus formed respecting 

 its structure, are confirmed at hand by the identity of 

 the other accompanying strata with those already enu- 

 merated in Seil. 



The island does not extend sufficiently to the west- 

 ward of this ridge to admit of the clay slate appearing, 

 as in Luing, on its western side ; but the whole eastern 

 part, with the exception of certain trap rocks and a small 

 portion of graywacke, consists entirely of that substance. 

 It has already been remarked, that the clay slate at the 

 southern extremity of Luing, which, according to the 

 order and position of the strata, is their eastern side, 

 abounds in a substance approximating to drawing slate. 

 The same circumstance occurs in Torsa ; a great portion 

 of its strata consisting of those black and soft varieties 

 already mentioned. They often contain mica, irregu- 

 larly and sparingly dispersed, and always disposed with 

 the flat sides of the scales parallel to the laminae of 

 the schist. This soft schist, after long exposure to 

 the weather, acquires in the interior a rusty brown aspect 

 and becomes tender; the surface turning grey and pul- 

 verulent, like those rocks into which felspar enters as 

 a constituent, I may here remark, that if the clay slate 



VOL. II. L 



