Vol. 60.] MOINE GNEISSES OF THE EAST-CENTRAL HIGHLANDS. 403 



2 feet, or even more. This increase and decrease, in different bands, 

 of evenly-disseminated brown mica imparts different shades of grey, 

 pale-grey, or pinkish-grey to the banded gneiss, and is the principal 

 cause of the evenly colour-banded aspect, which is its most 

 characteristic feature. The bedded aspect of the series is often 

 intensified by the arrangement of the individual crystals of 

 biotite parallel to the colour-banding. But it is still further 

 accentuated by the presence of films of felted dark mica, which 

 are always rigidly parallel, and appear on a cross-fractured surface 

 as fine black lines. 



These films decompose more readily than the rest of the rock, 

 and give rise to planes of diminished coherence, so that when 

 fragments become detached from a scar-face they break away along 

 those parallel surfaces. Further, this decomposed material weathers 

 out, leaving a series of minute parallel grooves that have the 

 appearance of dark lines when seen from a distance of a few feet. 

 It is, indeed, to the presence of these films that the flaggy weather- 

 ing of the Moine Gneisses is essentially due; and when the gneisses 

 occur in thicker bands, or the films are much farther apart, the 

 flaggy character is partly lost. It will be shown later that the 

 presence of these films is of the utmost importance in tracing these 

 rocks when they thin away to the south-east. 



That these rocks are altered sediments, and that the colour-banding 

 is coincident with the original bedding, is, in many cases, perfectly 

 clear from their chemical and mineralogical composition ; but, if 

 any further proof were wanted, it is to be found in the small cross- 

 cleaved, highly-micaceous bauds, originally more of the nature of 

 shales, that occur at intervals throughout the whole of the 

 Struan section. This cleavage of the original shale-material 

 obviously took place prior to any crystallization, and, as a rule, it 

 ends abruptly against the colour -banded rocks, which, fr&m their 

 present composition, must have been of a more sandy nature 

 originally, and would not cleave. The phenomenon is identical with 

 that observed so often in cleaved and folded Silurian rocks, although 

 the latter have not since been crystallized. Equally important, from 

 this point of view, is the occurrence in the deep cutting at the 

 Perth 42-milepost of a special type of grey gneiss, in which there 

 is scarcely any parallel banding ; even the parallel arrangement of 

 the biotite in the rock is not well marked, and the felted films of 

 biotite are entirely absent. This rock differs from the more common 

 type of gneiss in its mode of weathering, and on open ground forms 

 rounded blocks of massive aspect, somewhat resembling a very fine 

 granite. The absence of any indication of the original bedding- 

 suggests that the material was deposited under somewhat different 

 conditions from those of the parallel-banded gneisses. It may be 

 here noted that no thick band of such material ever occurs near 

 the south-eastern margin of the Moine Gneisses, or in the ground 

 where they end off. 



It may not be out of place, in concluding this account of the 

 macroscopic character of these gneisses, to draw attention to the 



