SKY. GEOLOGY. 295 



in this respect all the characters of an inland river as 

 far as the contraction extends. We may perhaps indulge 

 our conjectures still further in supposing that Sky was 

 once united to the mainland by means of this alluvium, 

 and that the gradual effect of the tide circulating through 

 the bay on each side had at length produced the effect 

 in question ; an effect not at all inadequate to its powers, 

 and of which parallel examples occur in the lateral action 

 of rivers on the alluvia of valleys ; as on the banks of the 

 Tay and in many others of the principal rivers of Scotland. 

 Greater effects have often been attributed to the corrosive 

 powers of the sea. I may remark that the narrowness 

 of the channel, which in one part does not exceed a quarter 

 of a mile, and the shallowness of the soundings compared 

 with the depth of those which separate the other parts 

 of Sky from the mainland, are friendly to this supposition. 

 These scarcely exceed ten fathoms in the middle, although 

 there are some deeper holes on each side ranging to thir- 

 teen, the bottom being every where gravelly, as if, like 

 the banks, it was the remains of some former alluvium. 



THE topography of the rocks of this island has been 

 already noticed, and may be deduced with sufficient 

 accuracy from the preceding remarks and from the map. 

 The order in which they may be observed to follow each 

 other may be described in a general way in few words. 



To gneiss, as the lowest rock, succeeds an alternation 

 of graywacke slate, red sandstone, compact grey sand- 

 stone or quartz rock, and common white quartz rock ; 

 the gneiss in one part of its course passing by an insen- 

 sible gradation into this series. After that follows a shelly 

 limestone, alternating with shale, and with a crystalline 

 limestone resembling those of primary formation. To this 

 there appears to succeed again the alternation of red 

 sandstone and graywacke schist described above, followed 

 once more by the same shelly limestone and shale, eacfy 



