410 31R. G. BARROW OX THE MOINE GNEISSES [NOV. I904, 



The latter was first seen in the railway-cutting" below the Perth 42- 

 milepost, and later on at the Perth 44-milepost. Mr. Macconochie 

 found it in the bed of the river at Struan and in a number of other 

 places, but it does not seem to be associated with the more massive 

 gneiss. This band, which we propose to call the ' Blotch-Rock/ 

 can be immediately recognized ; indeed it has .been met with 

 over a large area, and serves to show more than anything else that 

 the rocks composing the colour-banded gneisses were originally 

 quite thin. 



Area north of Struan, about the watershed of 

 Perth and Inverness. 



As we approach the watershed, the south-easterly dip slowly 

 changes, and becomes northerly. In the Gaick Burn, although the 

 typical grey gneiss is present, most of the bands are thin, and of 

 the type of those seen at Dalnacardoch. Three varieties of these 

 thinner bands are worthy of special notice. One is a kind of spangled 

 gneiss, and contains a considerable amount of biotite, and at times a 

 few crystals of muscovite at right angles to the banding (11,055). 

 From its mode of occurrence, we may assume that it was almost 

 certainly a shaly rock originally, but its dominant constituent now 

 is well-crystallized microcline, which forms 60 per cent, of the rock. 

 There can be little doubt that this microcline in such a rock 

 results mainly from the action of finely-divided micaceous material 

 on finely-divided quartz, thereby forming microcline ; for it will be 

 seen later that this excessive amount of microcline, in many cases, 

 characterizes the limestone and the adjacent shales. A specimen, 

 on the other hand, with much white mica contains little alkali- 

 felspar, suggesting that in this case no such interaction took place 

 (11,058). Associated with these, is a little band containing epidote 

 along certain lines. But one of the most striking features of the 

 exposures about the Gaick Burn is the frequent repetition of the 

 little ' Blotch-Hock,' showing that over a large area we do not move 

 more than a few feet from one horizon in the original sequence. 



Near the watershed the more thinly-banded rocks slowly dis- 

 appear, and the normal grey gneiss is again met with. The most 

 abundant phase (ll,052j is rather light-grey in colour, with quartz- 

 grains, somewhat rounded 'quartz-blebs' set in a matrix of felspar, 

 most of which is the typical microcline (97). Some plagioclase is 

 present, mostly decomposed. A somewhat darker phase (11,053) 

 contains slightly less microcline, with more quartz, and in this the 

 ' quartz-bleb ' structure is not so well shown. In both, the mica 

 shows the usual parallel arrangement ; the typical small sphenes 

 are fairly abundant, while minute zircons are more numerous than 

 in most of the specimens from the Garry area. 



(6) Area East of the Glen-Tilt Igneous Complex. 



In the Perthshire portion of the area east of the Glen-Tilt 

 eomplex, several important differences in the Moine Gneisses are 



