160 Dr. Mac Culloch's Supplement to the 



for the mica that characterizes the regular varieties. By degrees 

 the chlorite schist becomes predominant, and at length the felspar 

 is excluded, so that all appearance of gneiss ceases and a simple 

 series of chlorite schist remains. I suppress a detail of the endless 

 varieties found through this series, as such substances can rarely be 

 rendered intelligible in description. But I may add that hornblende 

 schist, so generally found to accompany gneiss, alternates here also 

 with it under many different aspects. 



With respect to the position and boundaries of this series, it is 

 found occupying beds of which the elevated edges present a rec- 

 tilinear direction towards the north-east, dipping to the eastward 

 in an angle which varies between 30 and 50 degrees. Its boundary 

 towards the west lies near the small island Oransa, where it is 

 succeeded immediately by the graywacke schist and the accom- 

 panying quartz rock or hard sandstone which were described in 

 the original paper, but which I shall presently describe again in 

 greater detail, having had an opportunity of verifying much of that 

 which was only conjectural, and of extending its limits to a much 

 greater distance than I had foreseen. 



Although the boundary of this series, in which gneiss and 

 chlorite slate form the principal parts, is thus defined at the northern 

 end of its western side, no such decided change is perceived at the 

 southern end of the same line, which, if protracted from the place 

 first mentioned near Isle Oransa, would cut a point on the western 

 side of Sleat. The interior of the country is too much encumbered 

 with peat and with vegetation to permit of any decision on a 

 subject so obscure as is the point of change between the gneiss 

 series and the rocks which follow it, and I must therefore limit 

 myself to the appearances which occur on the sea shore, where 

 every change can be traced in the most minute manner. Here 



