440 ME. G. BARROW ON THE MOINE GNEISSES [Nov. I904, 



too large to leave any doubt as to its horizon ; but on the north 

 side the infolds, which occur at the south-western foot of Sron na 

 Macranach, are so small that they can be identified only by the 

 aid of microscopic sections. One of these (11,137) is substantially 

 identical with another (11,136), taken from the south-west of the 

 Glen-Tilt complex, and lying between the Limestone and the 

 Quartzite. Both lie well across the belt of increasing crystal- 

 lization, and both contain a small quantity of sillimanite 



The evidence thus shows that here and there small patches or 

 infolds of Dark Schist and of the Main Limestone may be found 

 within the main area of the Moine Gneisses ; but, as previously 

 explained, there is now a tendency for the Dark Schist to become 

 more siliceous and to contain less dark dust, so that it is difficult to 

 identify. But, by first studying the more siliceous phases where 

 the Main Limestone is present to fix their position, such as those 

 seen at Derry Lodge, the true horizon and meaning of these infolds 

 become clear. 



Attention has already been drawn to the fact that a great deal 

 of the most highly-quartzose Moine Gneiss is simply the Central- 

 Highland Quartzite in a Moine-phase ; and if this, as well as the 

 infolds just referred to, be deducted from the areas in which 

 the Parallel-Banded rocks occur, it is soon seen that the true 

 Moine Gneisses need have had no great thickness originally. The 

 evidence of the incessant recurrence of some easily-recognized 

 small band in a typical area strongly supports this idea. 



(/) Slight Reversions to Similar Conditions 

 of Deposition. 



Attention was drawn to the fact that in Glen Mohr a change in 

 composition in the Honestones takes place in a south-easterly 

 direction, on similar lines to that seen in a north-westerly, as 

 the main area of Moine Gneiss is approached. But the change 

 is more local, and not carried to the same extent. In Glen Ey, 

 also, the Honestones become locally more siliceous and, moreover, 

 are occasionally mingled with the Pink Felspathic material, evenly 

 disseminated through them. Similar small local changes occur in 

 other areas ; and an excellent example occurs in Glen Loch, in the 

 upper part of Glen Pirnate, in Perthshire. Here, on the margin 

 of the Quartzite, a thin band of material occurs (3838, 3839), 

 practically identical with that seen in the less-altered rocks of 

 the Unich section (PI. XXXVII, fig. 2, No. 150). These small 

 reversions are of considerable importance, as they afford the 

 key to the phenomena of the distribution of the Parallel-Banded 

 material. This distribution has resulted from the natural vanning 

 of the detrital material brought down hy a large river with many 

 mouths, of widely-different sizes, but all tapping a common source 

 at A (see fig. 8, p. 441). There must be a series of points in 

 front of these streams, at which clastic material of a definite 



