Il8 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [May 1 894, 



portion of the district is gneissose, but as we proceed in a south- 

 easterly direction towards the Highland border the rocks become 

 finer in grain and more like the ordinary schists, finally assuming 

 the phase of phyllites and of more or less crystalline arkose grits. 

 This variation in the character of the rocks is accompanied by a 

 change of the constituent minerals, more especially the aluminous 

 silicates. It has been observed, for instance, that sillimanite, 

 cyanite, and staurolite characterize three more or less distinct zones, 

 which seem to be dependent on relative proximity to the igneous 

 masses, since the zones do not necessarily coincide with the strike 

 of the rocks. Thus, a thin bed of quartzite, which retains its 

 character in consequence of the simplicity of its chemical com- 

 position, may be followed through all the zones, whereas the bed 

 adjacent to it is : — in the outer zone a staurolite-schist, in the 

 intermediate zone a cyanite-gneiss, and near the contact with the 

 igneous rock a coarse sillimanite-gneiss. With regard to the two 

 latter minerals, both of which have essentially the composition of 

 andalusite, it may well be that the accession of heat alone, as 

 pointed out by Mr. Teall, would be likely to convert cyanite into 

 sillimanite, and thus the line separating these two minerals in that 

 area might, to a certain extent, be regarded as an isothermal. 

 With respect to staurolite, the Author allows that the zone of this 

 mineral very nearly corresponds with the actual outcrop of a 

 particular bed. In this case the original composition of the rock 

 may have had something to do with its development. On in- 

 specting the map which accompanies this paper, one cannot fail to 

 observe that these three zones present a marked parallelism with 

 the great mass of ' newer granite ' on the confines of the counties 

 of Forfar and Aberdeen ; but the Author is of opinion that the 

 metamorphism produced by these later granites can be easily 

 distinguished from that v .which he has described, its effect having 

 been to destroy many of the characters due to the earlier action. 



Mr. Barrow considers that the sedimentary character of the rocks, 

 as a whole, is established by their chemical composition. Lime- 

 stones, shales, quartzites, and coarse grits may all be recognized in 

 the metamorphic area. The lowest rocks, the quartzites of the 

 North Esk Valley, are highly siliceous, though containing a certain 

 amount of felspar, which both in the grits and gneisses is stated to 

 be almost exclusively oligoclase. Even of those rocks whose origin 

 may be regarded as more obscure there is nothing, he says, to 

 suggest that they have been formed of crushed igneous material. 



