Muscovite-biotite-gneiss in South-eastern Highlands. 145 



3. " On an Intrusion of Muscovite-biotite-gneiss in the South- 

 eastern Highlands, and its accompanying Thermo-metamorphism." 

 By George Barrow, Esq., E.G.S., of the Geological Survey. 



The area to which this paper refers lies in the north-eastern part 

 of Forfarshire, and is drained by the two Esks. The author first 

 describes the distribution, mode of occurrence, and petrological 

 characters of the intrusive masses. In the north-western portion of 

 the area the intrusive rock is always a gneiss, and occurs in thin 

 tongues which permeate the surrounding rocks. Towards the south- 

 east these tongues amalgamate and form large masses, in which the 

 foliation is less marked. Moreover, in this direction the large 

 masses are often fringed with pegmatite, especially on their southern 

 and eastern edges. Where the rock is a gneiss, it is composed of 

 oligoclase, muscovite, biotite, and quartz, but contains no microcline. 

 As the gneissose character becomes less marked, the oligoclase dimin- 

 ishes in amount, and microcline begins to appear, especially towards 

 the margins of the masses. In the most south-easterly of these 

 microcline is greatly in excess of oligoclase. 



The differences in structure and composition of these masses are 

 believed by the author to be due to the straining off of the crystals 

 of earlier consolidation during intrusion under great pressure. The 

 still liquid potash-bearing portion of the magma was squeezed out 

 and forced into every plane of weakness in the surrounding rocks ; 

 and that portion of it which contained the highest percentage of 

 potash finally consolidated as pegmatite. 



Special attention is directed to the distribution of pegmatite. 

 This rock is widely distributed in the Southern Highlands, and cuts 

 across every known system of folding. It is consequently newer 

 than any member of the metamorphic series. 



The surrounding metamorphic schists are next dealt with. These 

 are remarkable for their highly crystalline condition, and also on 

 account of the presence of many minerals known to occur in regions 

 where thermometamorphism has taken place. The characters of the 

 more important minerals are described in detail. The rocks of the 

 metamorphic area become less and less crystalline as they are 

 followed towards the Highland border. Three zones, characterized 

 respectively by the minerals sillimanite, cyanite, and staurolite, have 

 been roughly mapped. The more important rocks found in these 

 zones are described in detail, and evidence is given to show that the 

 boundaries between the zones do not in all cases 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 composition, 

 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. Evidence is given to show that the original rocks 

 formed a sedimentary series. 



The phenomena are compared with those of other areas where 

 thermometamorphism has taken place ; and the conclusion is reached 



Phil. Mag. 8. 5. Vol. 36. No. 218. July 1893. L 



