418 ME. G. BAKK0W ON THE MOINE GNEISSES [Nov. I904, 



position for some distance. Immediately below the big bend at 

 Aucbgoul it changes suddenly in composition, in a manner that 

 seems to exclude the probability of an igneous origin. 



The big bend just mentioned is cut in Drift, but at its northern 

 end, the Limestone, repeated several times by folding, is seen either 

 close to or touching the Moine Gneiss. One outcrop of limestone is 

 so much purer, and more like the normal Blair-Atholl Limestone, 

 that it seems at first difficult to believe that we are still dealing 

 with the same bed. 



Opposite the mouth of Glen Mhaire the river once more flows 

 along the strike of the rocks, and the Moine Gneisses are now seen 

 to be rather more quart zose and more finely -banded. In the bank 

 close to and above the river are several infolds of the Limestone, 

 associated in one case again with the Pink Felspathic Rock. A few 

 patches of tough Dark Schist, almost a Moine Gneiss at times 

 (10,5-18), intervene between the Limestone and the Gneisses. In the 

 next long bend, immediately above Glen Mhairc, the river flows 

 exclusively over the Gneisses, which now lie to the north-west of 

 the last outcrop of the Limestone. Beyond this bend, to the north- 

 west, the Moine Gneisses stretch for many miles in an unbroken 

 sheet. 



A peculiar interest attaches to this section, for the curve of the 

 bend penetrates more deeply than usual into the belt of increasing 

 metamorphism ; and, comparing the rocks at the centre with those 

 at the two ends, the contrast, both in crystallization and thickness 

 of the bands, is well-marked. Moreover, if the steep bank at the 

 south-eastern extremity of this bend be ascended for a short 

 distance, the decrease in crystallization and the thickness of the 

 bands aie still better seen. A similar change may also be noted in 

 the small quarry close to the roadside, just at the commencement 

 of the next bend. 



A little faither on, in the river-bank, the Limestone is clearly seen, 

 lying in an eroded hollow in the now-attenuated representative 

 ot the Moine Gneisses ; while apparently above the Limestone is 

 the Dark Schist, but in reality this is a deception, the meaning of 

 which is explained on p. 431. Still farther up the Tilt, the slow 

 decrease in the crystallization of the gneisses may be noted, as fold 

 alter fold of the Limestone is crossed. Accompanying this change is 

 an alteration of the material of which the gneisses were originally 

 composed. It is obviously becoming more of the nature of a 

 sandy mud. 



"We now reach the famous section of the Glen-Tilt Marble-Quarry, 

 known to geologists since the days of Hutton, Playfair, McCulloch, 

 and Murchison, who noted its le^emhlance to certain limestones and 

 their associated rocks in the Korth-Western Highlands. 1 Here the 

 Limestone is folded again and again on itselt,so as to form a rather 

 thick mass; ne^t to it comes the little sill of hornblende-schist, 



1 See Quart. Journ. Geol. Soe. vol. xvii (1861) p. 223, second footnote. ,- 



