406 ME. G. BAEROW ON THE MOINE GNEISSES [NOV. I904,. 



fact that, their highly-crystalline character is shown, not by the 

 quartz or felspar, but by the persistently-large size of the rnicas,. 

 when these are present in notable quantity. Years of study have 

 proved that this is by far the most sensitive test by which to judge 

 of the degree of crystallization in altered sediments, such as were 

 originally normal sandstones and shales. 



Microscopic Characters of the Gneisses in the Struan 

 Area. — Great light is thrown on the structure and composition of 

 these gneisses by the aid of the microscope. Taking first the 

 prevalent type — the parallel-banded rocks, we find that they 

 are essentially felspathic gneisses, the felspar being usually in excess 

 of the quartz, and in some cases occurring almost to the exclusion of 

 the latter. As the quartz decreases in amount it tends to assume a 

 rather rounded form, embedded more or less in the felspar, and con- 

 stituting 'quartz-bleb structure.' It may, when present in very small 

 quantity, occur as minute globules in the felspar, imitating exactly 

 the micropoikilitic structure of igneous rocks. The felspar is of two 

 kinds — microcline, for the most part fresh and showing the typical 

 cross-hatching ; and plagioclase, usually much decomposed. It is 

 almost impossible, in many cases, to be certain of the nature of the 

 plagioclase-felspar, but in some instances it is clearly oligoclase. 

 The relative proportion of microcline to plagioclase in the gneisses 

 of the Struan area varies greatly. In the lighter-grey varieties 

 plagioclase seems to be, as a rule, slightly in excess : in the darker- 

 grey varieties, microcline often exceeds the other in amount. AVhen 

 the gneiss weathers with a distinctly-pink edge, the microcline seems 

 usually to be the dominant felspar in this area; and it is, at 

 times, more abundant than plagioclase and quartz taken together. 

 A good idea of the general structure may be obtained by selecting 

 a specimen in which the quartz, microcline, and plagioclase are 

 present in nearly-equal proportions. 1 It will be seen that the 

 grains are, on the whole, evenly distributed, as if they had been 

 first mixed in a pepper-pot and then shaken out. This granular 

 arrangement of the component grains may be described as a 

 'granulitic structure"; but it cannot be too clearly understood 

 that it is unlike the granulitic structure of many of the schists 

 of the Southern Highlands. The microcline usually retains this 

 granular mode of occurrence, even when present in large quantity, 

 and it rarely helps to bring out the foliated character or parallel 

 structure of the rocks. But, if the plagioclase increases in pro- 

 portion, it assumes irregular forms, and tends to occur in much 

 larger and often elongated patches that help to define the parallel 

 structure. The quartz in the latter case frequently appears 

 embedded as ' blebs ' in the felspar, a mode of occurrence that can 

 oft* n be made out by the aid of a hand-lens. 



The foliated aspect, in hand-specimens, of a single band or flag 



1 See PI. XXX7V, fig. ] (No. 88). The low numbers refer to photographs in 

 the possession of the Geological Survey : the high ones (10,422) to the micro- 

 scopic rock-sections. 



