426 MR. G. BARROW OX THE MOINE GNEISSES [Nov. I904, 



staurolite appears, and twin-chlorite and other magnesian silicates 

 steadily increase in amount. Where the metamorphism is more 

 intense, the same phenomenon is shown by the greater abundauce 

 of cordierite in the lower part of this bed, while andalusite is more 

 abundant in the upper. 



Another characteristic of the zone a little above the Twin- 

 Chlorite Rock, is the presence in large quantity of a felspar proved by 

 Dr. Teall to be of the oligoclase-andesine group, containing curving 

 lines of dark dust (the 'Felspar-Rock'). It occurs, to a small extent, 

 through most of the bed, but appears to be abundant only towards 

 the lower part. Its distribution seems to be the same as that of 

 the tremolite-rock ; the two, so to speak, go together, and have been 

 recognized as far away as the neighbourhood of Druid Farm, 

 north-west of Ben Yrackie (10,777). The dark dust that occurs in 

 this felspar is met with throughout the whole of the Dark Schist in 

 the Braemar area, but as a rule is most abundant about the Twin- 

 Chlorite Rock. Here, a small portion of it is undoubtedly graphite, 

 though, in most cases, very little of this material is of that nature. 

 The dark dust seen under the microscope is often in part leucoxene, 

 but by far the greater portion of it is iron-ore ; its real origin was 

 suggested by Mr. A. Dick's examination into the cause of the blue 

 colour of unweathered London Clay. This proved to be the pre- 

 sence of a large number of minute spheroids of marcasite, and there 

 can be little doubt that the dark dust of these rocks had a similar 

 origin. The iron-ore in these rocks is slightly magnetic, and, if 

 a specimen be ground to very fine powder, the greater part of the 

 rock can be picked up with a magnet, owing to the even dissemination 

 of the iron-ore throughout it. 



5. The Main Limestone. — The typical Main, or Blair- Atholl 

 Limestone, is well seen at the southern end of the corrie opposite 

 Newbiggin in Glen Clunie, where it possesses the characteristic pale 

 bluish-grey colour and crystalline aspect. The Clunie area shows 

 well the tendency of the rock to become more impure as it approaches 

 the belt along which the Moine Gneisses set in ; or where there 

 is a hiatus in the succession, and parts of the beds are missing. 

 As a rule, however, only the basal portion is markedly impure, 

 and as (in many cases) it is this part repeated by folding that is 

 really seen, it gives the erroneous impression that the whole bed is 

 impure. 



6. The Calc-Flintas, or Parallel-Banded Calcareous 

 Shales. — This bed consists of thin laminre alternately richer and 

 poorer in lime. The peculiarity from which it takes its name, is 

 its more or less persistent flinty aspect, due to the presence of a 

 variable number of bands composed of quartz, biotite, calcite, 

 pyrites, and leucoxene. These originally contained much quartz 

 and clastic chlorite in a line state of subdivision, which, when 

 heated, form a kind of hornfels at a specially-low temperature 

 and this hornfels is so intensely hard, that it resisted shearing 



