54 Dr. M'^Culloch's Supplement ari/ Remarks on Quartz Rock. 



The character of the gneiss is here as decided as its extent is unquestion- 

 able ; since it forms a very large portion of the country, and presents all its 

 varieties from the granitic to the schistose. And although its structure is such 

 that it cannot always be satisfactorily distinguished from micaceous schist^ yet 

 these cases bear no proportion to those in which its character is strongly 

 marked ; while, over a large space, it presents that approximation to the gra- 

 nitic disposition whicii characterizes the gneiss of the Western Isles. It is also 

 distinguished by the presence of granite veins, and by the occasional occur- 

 rence of very extensive masses of granite. 



This great mass of gneiss is succeeded in some places by the tract of quartz 

 rock formerly mentioned, and alternates with it in others. Thus the latter 

 rock is found under a great variety of position ; sometimes in a detached state, 

 occupying the summits of those mountains of which the chief mass consists of 

 gneiss ; at others, covering considerable tracts of country, and forming the 

 lowest as well as the highest land. Examples of the former will be found in 

 Ben Stack and in the northernmost Ben More ; while the great tract from 

 Ben More Assynt to Canasp and Cuniach, presents an instance of the latter 

 disposition. But this change is not always effected without an alternation of 

 the two; not merely that slight alternation of a few beds which so often occurs 

 at the point of contact between approximate rocks, but one that involves 

 masses of almost mountainous bulk. 



It is necessary to remark, that the leading direction of the beds of gneiss, 

 as well as of the rocks which accompany it in this tract, is north-easterly ; that 

 direction, however, from the usual inconstancy and undulation of the beds, 

 being seldom determinable within four or five points of the compass; since it 

 is subject to vary twenty degrees or more from this average bearing. The dip 

 is still more various, but may be generally comprised between the angles of 

 ten and thirty degrees. In both the places which I shall point out as present- 

 ing the alternation in question, the direction appears to be E.N.E. and the dip 

 about fifteen degrees. 



One instance of alternation is seen at the north side of Glen Dhu, and near 

 its upper extremity. The bed of quartz rock appears to be about one hundred 

 feet thick, and reposes on tiiat gneiss which extends to the western sea. It 

 presents its broken face to the southward, and is easily examined. It consists 

 of very even and parallel .strata, of an extremely compact variety, subject to 

 crack in numerous directions from the injuries of the weather, and then 

 crumbling so as to form a slope extending from the foot of the precipitous face 

 to the shore of the Loch. Its predominant colours are white, yellowish, and 

 gray; but many of the beds are of a pink, others of a brown purple hue; 



