SKY. GEOLOGY. GNEISS. 297 



' At one extremity the gneiss is defined by a line com- 

 mencing near a small indentation marked in the map oppo- 

 site to Isle Oransa : it will hereafter be seen that at the 

 opposite end of that line the boundary cannot be fixed ; a 

 circumstance indicated in the colouring of the map. This 

 line cannot be traced consistently even through the extent 

 thus defined ; the accumulation of soil, and other causes, 

 preventing a view of the rocks in many parts of the space 

 over which it extends : but as the indications of a recti-* 

 linear course are every where predominant, and as the 

 actual boundary is often visible, I have without scruple 

 defined it by a precise line to the eastward of all those 

 points where its limit has been actually traced. 



This line tends north-easterly, and will be found to 

 regulate the direction of the edges of the gneiss beds 

 wherever they are sufficiently persistent and elevated to 

 give indications of their course. Their dip is toward the 

 south-east, and will be found most constant and uniform 

 wherever the rectilinear direction is most perfect. In 

 one or two instances they subside to a very low angle 

 or become actually horizontal, in a very few cases they 

 are slightly displaced or bent, and in one or two only 

 I observed the dip to be reversed. The angle of elevation 

 varies so much in different places that no rule of general 

 application can be given. Angles varying from 30 to 40 

 appear to be the most frequent, but towards the south- 

 western extremity of Sleat they occasionally become 

 vertical. Some of the minor disturbances and contortions 

 seem to arise from the trap veins, which are abundant, 

 traversing the gneiss in every direction. The others 

 must be referred to the causes that have influenced the 

 general arrangements of this rock. 



The laminar structure prevails in the gneiss of Sky, 

 which is identical with a very large tract of the same 

 rock to be found in Glen Elg on the opposite mainland. 

 Not only are the beds regularly disposed in parallel 

 planes, but the structure of the rock corresponds to 



