408 MB. G. BABEOW ON THE MOINE GNEISSES [Nov. I904, 



serve, more than any other members, to correlate the Moine Gneisses 

 of different areas despite local variations of character. It has 

 been found in the Struan and adjacent areas that these rocks 

 are especially rich in microcline. Moreover, when the pink colora- 

 tion is well marked, they usually contain in addition some calc- 

 silicate, which is most commonly epidote or zoisite, but at times 

 hornblende is present. Typical examples of the epidote-bearing 

 variety are abundant in the second cutting above Struan Railway- 

 station. The most interesting example of this pink type, however, 

 occurs in the bed of the Garry, immediately in front of Dalnacardoch 

 Lodge. Like all the rocks close by, it splits into comparatively-thin 

 slabs, owing to the presence, at short intervals, of the thin films of 

 felted biotite already mentioned. Between these films the rock is 

 not particularly fissile ; indeed, it is rather tough, and shows a 

 mottled red-and-green coloration, on a cross-fractured surface. A 

 section shows that it is composed mainly of the typical cross-hatched 

 microcline. This forms a kind of groundmass, in which are set a 

 number of aggregates of green mica, the long axes of which are 

 parallel to, and, indeed, serve to mark, the foliation of the rock. 

 A small amount of plagioclase (in irregular patches) and a little 

 white mica are also present. Apatite is fairly common, and occurs 

 in much the same manner as the quartz. This latter mineral is 

 present in very small quantity, and most of it is found as tiny blebs 

 in the microcline, affording a perfect example of micropoikilitic 

 structure. The occurrence of this rock, so rich in alkali-felspar, is 

 especially interesting, as it tends to recur again and again over a 

 very large area, and apparently at a definite horizon. 1 



The highly-micaceous Gneisses. — These rocks are charac- 

 terized by abundant white mica and biotite or chlorite. For the 

 most part they are cross-cleaved, as already stated ; but where only 

 a very thin parting occurs, the gneiss is at times ' rodded,' that is, 

 the micas are all elongated in a definite direction, and there is no 

 specially-marked plane of schistosity. This variety serves to show 

 that the originally-softer parts of the series have often suffered 

 considerably from dynamic action prior to crystallization. 



These micaceous bands possess a somewhat different structure 

 from that of the other gneisses. In the cross-cleaved variety there 

 is a tendency to form lenticles, free from mica, as in the true 

 schists, but the lenticular structure visible in the hand-specimen is 

 not nearly so obvious under the microscope. Comparatively-little 

 microcline is present, and only a moderate amount of felspar. 

 Quartz, on the other hand, is more abundant than would have been 

 suspected. Apatite is much more common than in the parallel- 

 banded rocks. The abundance of quartz explains, what is specially 

 noticeable, the total absence from the micaceous gneisses of silicates 

 of alumina, such as sillimanite, cordierite, andalusite, etc. Clearly, 



1 See PL XXXVI, fig. 1 (No. 84). Further investigation has shown that 

 this is the ' Pink Felspathic ' rock described on p. 416, and marks the top of 

 the Moine Gneisses. 



