128 SEIL. GEOLOGY. 



observed, skirting some of the shores and surmounted 

 by the trap of that ridge ; the subjoined sketch being 

 consequently deduced from its position, and from general 

 considerations. It is indeed possible that some of the 

 beds of rock, which here, as in other places, alternate 

 with clay slate, may exist in lieu of that which I have 

 supposed to occupy the whole. If so, however, they 

 must be very peculiarly placed, as between Eysdill 

 and the bottom of the sinuosity which looks to the 

 south-west, the edges of almost all the beds of schist 

 that occupy the tract surmounted by the trap, are acces- 

 sible. The direction of these beds, as determined by 

 that of their elevated edges, is N. E. by N. ; a bearing 

 which corresponds to those of the leading ridges as well 

 as to the outlines of the island, and is very evidently the 

 cause of both. The dip also is easterly, and in both 

 these circumstances the strata of Seil correspond with 

 those of Kerrera, as far as the latter admit of compa- 

 rison; and more exactly still with those of many of 

 the islands of this division hereafter to be described. 

 The quantity of the inclination is here, as every where 

 else, inconstant; ranging in a general way from twenty 

 to fifty degrees. 



The mineral character of this rock is too well known 

 to require description, since, like the schist of Kerrera, 

 it is identical with that so largely quarried in the adjoin- 

 ing island of Eysdill, which is in fact but a detached 

 portion of these very strata. The peculiarities which it 

 presents are here passed over, because they are noticed in 

 some other of the numerous situations in which it occurs. 



Two substances sufficiently distinct in mineral character 

 from common clay slate, are here found interstratified with 

 it, but, as far as I saw, in very inferior quantity. One 

 of them is an argillaceous schist slate highly charged with 

 minute particles of quartz, so as almost to resemble a sand* 

 stone, and, like some of these, crumbling under the 

 hands into a blueish sand. The other is a perfect gray- 



