18 Mr. Macgillivray on the Granite of 



Fig. 1. represents the general appearances of the protuberance^ 

 on the summits of the granitic mountains. Fig. 2. represents a 

 portion of a columnar mass at the upper part of the corry of Loch- 

 na-gar. 



Granite has been described as composed of concretions, the in- 

 tervals between which consist of softer materials. When it is ex- 

 posed to the action of the weather, the harder portions emerge from 

 the rubbish, and assume a spherical form. The Braemar granite, 

 in becoming disintegrated, never presents spherical forms, but re- 

 solves itself into tabular masses, more or less rounded on the edges 

 and angles. Without supposing any concretionary structure in it, 

 this phenomenon is easily accounted for. If the granite had been 

 originally at a high temperature, it would, on cooling, have crack- 

 ed into seams more or less parallel, one set at right angles to the 

 other. The atmosphere acting upon the surfaces of the cracks, 

 would produce the tabular fragments. The same effect might re- 

 sult from the accidents of a mass originally liquid. At all events, 

 the Braemar granite is apparently uniformly granular in its mass, 

 and although portions may occur harder than others, I have found 

 no evidence of a concretionary structure in it. Fragments of any 

 size, having a rounded form, occur only in the beds of streams, and 

 on the slopes, where the phenomena may be accounted for by more 

 obvious causes than an original disposition of the rock into harder 

 masses separated by less coherent portions. 



Drusy cavities occur in the granite, but in what proportion, or 

 of what size, I am unable to say. One, which I observed in the 

 face of a rent in the precipice of Ben-na-buird, was about a foot 

 in diameter, and presented the constituents of the granite regular- 

 ly crystallized. A few simple minerals, as topaz and beryl, are 

 said to have been found on these mountains. 



Precipices and Carries. — The corries, or semicircular hollows of 

 these mountains, and their long precipices of irregular height and 

 direction, present the most remarkable geological phenomena con- 

 nected with them. There are various ways by which they might 

 be accounted for ; but it is first necessary to describe them. The 

 first feature is a lake of nearly circular form, and unknown depth. 

 Around this lake, on one side, and at a greater or less distance 

 from it, rises a semicircular range of precipices, cracked and broken 

 into ridges and shelves, sometimes perpendicular, more commonly 

 sloping. These precipices, from their base, at a height of from 100 

 to 500 feet above the lake, send down to the latter a curved slope 

 of blocks, stones, and gravel. Toward the valley or slope of the 

 mourftain, the lake is bounded by low irregular ground, in which 

 it has found a tortuous passage for its waters. 



The accompanying diagram. Fig. 3. will afford some idea of the 

 form of these corries : — a, the lake ; b, the overhanging precipice ; 

 c, the slope of debris ; dj d, distant parts of the semicircular pre- 

 cipicej with their slopes. 



