March, 1894. THERMO-M ETAMORPHISM IN SCOTLAND, ujcj 



and plane of weakness in the surrounding rocks. This results in 

 leaving behind large masses of rock composed of what are known as 

 the " earlier crystals of consolidation," while the residual liquid is 

 forced upwards and outwards, finally consolidating as extremely 

 coarsely crystalline veins and masses known as pegmatites, or " giant 

 granite." A peculiarity of the present instance is the enormous distance 

 to which these veins extend from the parent mass. Perhaps no case 

 has before been clearly established in which pegmatites extend so far 

 as a mile from the surface outcrop of the parent mass of any granite 

 of post-Cambrian age ; but in the area under consideration these 

 veins extend as far as 40 miles from any exposure of the muscovite- 

 biotite gneiss. 



These pegmatites throw great light on the sequence of the rocks 

 in this intensely-folded area. Obviously, if by a great normal fault, 

 rocks which are penetrated by igneous veins are brought against 

 rocks which these veins have failed to reach, the permeated rocks 

 must be the lower of the two, and we are thus able to show that one 

 group of rocks is higher or lower than another. A starting-point of 

 this nature is invaluable in an area where it is difficult to say if any 

 dip is true or reversed. It may be here noted that the characteristic 

 feature of the pegmatites is the presence of large crystals of musco- 

 vite. The original magma consolidates normally as a true granite 

 containing white and brown mica, but the pegmatites never contain 

 large crystals of brown mica. This leads to the natural inference 

 that the pegmatites bearing brown mica, such as may be seen at Cape 

 Wrath, are the upward prolongations of a granitite magma, or one 

 which contained brown mica only and not white mica, and which has 

 been injected under precisely similar conditions to those of the south- 

 eastern Highlands. 



Most interesting is the development, over a vast area, of certain 

 minerals, such as sillimanite, cyanite, and staurolite, which have 

 already been described by many authors as the result of the intrusion 

 of large igneous masses into sediments. The development of these 

 minerals is found to be dependent on nearness to or distance from 

 the main intrusion, and it may be further added that the crystalline 

 condition of the entire region may be said to " rise or fall " away 

 from the main intrusion, just as the pegmatites do or do not reach 

 the surface. From these facts it may be inferred that the upward 

 permeation of igneous (often gneissose) material must have taken 

 place over a large part of the southern Highlands. In the ordi- 

 nary post-Cambrian granites, the development of certain minerals 

 occurs practically within cylindrical rings, one outside another, 

 surrounding the inner cylinder of granite. Here, however, we have 

 a new phenomenon to deal with. The thermo-metamorphism is 

 dependent, not so much or so entirely on the distance away from 

 the parent edge of the igneous mass, but rather upon the fact of 

 the rocks being at a greater or less height above what we may 



