56 Dr. M-^CuLLocH's Snpplemcntaiy Remarlcs on Quartz Rock. 



ultimately into pure quartz rock, of various degrees of compactness and 

 induration*. 



On a former occasion f I pointed out an alternation Avhich quartz rock forms 

 in the Isle of Sky with the red sandstone. That observation is no lonaer 

 solitary ; since the west coast of Sutherland also produces many instances 

 of the same nature, besides that at Glendhu just mentioned. As the descrip- 

 tion of the conglomerate and sandstone of that district is not my object in the 

 present communication, it is sufficient to say that they resemble the corre- 

 sponding rocks of Sky, above referred to, in every respect ; presenting the 

 same varieties in colour, composition, and induration. The alternation of 

 this sandstone, under its several modifications, with the quartz rock, is visible 

 in many places ; and when the larg-e-grained conglomerates form the alter- 

 nating bed, the appearance is not a little unexpected. 



These alternations occur in two ways, both in single beds and in mountain 

 masses; in the former case differing from the example described in Sky, 

 where no interchange of beds on a small scale was observed. The beds in 

 question, like those above enumerated, have a north-easterly course, with a 

 southern dip, frequently at angles not exceeding 10°. Among the places 

 where these alternations are visible the most accessible are Cuniach and Ben 

 Stack. To examine the former mountain, if the object is to trace the alterna- 

 tions on the great scale, a considerable tract must be compared; but in various 

 parts of the outskirts of the mountain the minor alternations are accessible 

 with little labour. Ben Stack presents these latter only. — It is unnecessary 

 to enter into more details of localities, as many of them cannot be particularly 

 described, for want of marks ; and the geologist who is inclined to undergo 

 the toil of traversing this trackless and abandoned region, will not regret the 

 additional labour of investigating more minutely the only objects which it 

 presents to reward him for the sacrifice. 



FURTHER REMARKS ON QUARTZ ROCK. 

 1818—1819. 



The object of these concluding remarks is to supply that which was want- 

 ing in the former supplements, and to complete, as far as the history of 



* These transitions are remarkable for the evenness and the parallelism of their fracture ; pro- 

 ducing slates very well calculated for roofing in windy climates ; but still more applicable to the 

 purposes of paving, since they afford slabs of ten or twelve feet in length, perfectly even, and 

 easily shaped by the hammer. f Geological Transactions, vol. iv. p. 273. 



