Vol. 60.] OP THE EAST-CENTRAL HIGHLANDS. 401 



west of the great Glen-Tilt igneous complex : (b) the area lying 

 between the Glen-Tilt complex and the great Cairngorm mass of 

 granite ; and (c) a tract which forms a small portion of the ground 

 to the south-east of the latter intrusion. These masses of granite 

 are chosen to fix the position of the areas, simply because they are 

 shown on most small-scale geological maps, and are easily 

 recognized. 



(a) The Moine Gneisses of the Struan Area. 



This area is bounded on the west by the Eiver Garry, and, as 

 the sections are easily accessible, it will be convenient to begin 

 with a description of the gneisses there exposed, and to use this as 

 a standard of reference in describing the gneisses elsewhere. 



The Garry Section. — Since the days of McCulloch, the liiver 

 Garry above Struan has been famous for the sections of flag-like 

 rocks which are exposed in its bed and banks, from Struan, almost 

 without interruption, to the summit of the Highland Kailway. Its 

 most striking feature is the extraordinary simulation of a normal 

 sequence of enormous thickness, the dip being apparently persistent 

 in one direction (the south-east), at an angle of from 20° to 30°. 

 As seen from a distance, almost the whole sequence consists of 

 well-bedded flags, the component bands varying in thickness, the 

 average of which is about 6 inches, or perhaps less. It will be 

 shown later that thicker bands predominate at one portion of the 

 series, and thinner at another. 



The imitation of a sequence of enormous thickness, and the perfect 

 preservation of the parallel banding, together with their highly- 

 crystalline condition, enable these rocks to be easily identified. 

 Since the days of McCulloch, many observers have noted the extra- 

 ordinary resemblance of the rocks to the flaggy gneisses of the North- 

 Western Highlands, now known as the Moine Gneisses. Further, 

 the mapping of the Highlands has progressed far enough to leave 

 no reasonable doubt that the Struan Flags and the Moine Gneisses 

 are one and the same group of rocks, and it is consequently 

 advisable to recognize this identity in describing them. Additional 

 facilities for their study have been afforded by the cuttings 

 recently made in widening the Highland Kailway above Struan, by 

 means of which fresher material can now be obtained for 

 microscopic examination. (See figs. 2 & 3, pp. 404 & 405.) 



The Grey Gneiss. — The dominant member of the Moine 

 Gneisses in this area is an evenly colour-banded and markedly- 

 granular, acid gneiss, containing a variable, but often considerable, 

 amount of brown mica. White mica is frequently present in 

 the typical gneiss, but as a rule in smaller quantity than brown, 

 while it is in many bands absent altogether. Though the amount of 

 brown mica varies incessantly, taking the group as a whole, it is for 

 the most part evenly distributed through small thicknesses of the 

 gneiss, varying from a fraction of an inch to sometimes as much as 



