IV. 



Thermo-metamorphism in the South-Eastern 



Highlands of Scotland. 



IN the north-east of Forfarshire, there is a tract of moorland, much 

 covered with peat and heather, that forms part of the singularly- 

 flat table-land of the south-eastern Highlands. It is drained by the 

 North and South Esk, and in the craggy sides of their valleys may 

 be seen exposures of the gneisses and schists of which the area is 

 chiefly composed. Glen Clova, to the south-west, is especially well- 

 known for its picturesque scenery. A detailed study of this area 

 shows that there are several masses of intrusive rock, gneiss, and 

 granite, which are probably connected underground, and that the 

 highly crystalline character of the surrounding schists is the result, 

 mainly, of thermo-metamorphism, or of alteration by heat rather than 

 by pressure and mechanical movements. This conclusion is of some 

 interest, as the special features may, after all, have no necessary 

 connection with great antiquity, but be due to the depth in the 

 earth's crust at which the metamorphism took place. It serves also 

 to strengthen Dr. Barrois' views that regional metamorphism and 

 contact metamorphism are much the same thing, differing not in 

 hind, but in degree .^ 



The ordinary condition of the intrusive rock is that of a slightly 

 foliated granite, with two micas : muscovite or white mica, and 

 biotite or brown mica. Hence the rock may be termed a Muscovite- 

 biotite Gneiss. Intruded among the schists are a number of 

 pegmatites, or veins of coarse granite, the origin of which was 

 uncertain ; but they have now been traced to the parent mass of 

 granite rock — the muscovite-biotite gneiss. 



In considering the effect of the forcing of a granite magma, under 

 enormous pressure, into the surrounding rocks, we have practically to 

 deal with a process akin to that by which silver is extracted from 

 lead. Just as the silver crystals are strained off" from the lead at a 

 certain temperature, so certain crystals are strained off" from the 

 granite-magma during the process of forcing it through every crack 



1 General results of paper " On an Intrusion of Muscovite-biotite Gneiss in the 

 South-east Highlands of Scotland, and its accompanying Metamorphism." Quait. 

 Joitni. Geol. Soc, vol. xlix., pp. 330-358, pis. xv., xvi., 1893. 



