210 E. Greenly — Diffusion of Granite into Schids. 



3. Gneisses and Granites of Eastern Sutherland. 



The plienomena of Eastern Sutherland were described some years 

 ago in a joint paper by Dr. John Home and myself.^ The granites, 

 which are generally foliated, lie as sills in a region entirely composed 

 of gneissose rocks in which no original structures whatever have 

 been detected. On parts of the northern coastline, where granulitic, 

 somewhat siliceous rocks prevail, the granitoid matter is injected 

 "lit par lit," producing complex synthetic banded gneisses. But 

 on other parts of the coast, and inland about Kinbrace, where a flaky 

 or wavy biotite gneiss is the dominant type, we find the permeation 

 phenomena. 



In " lit par lit " injection the injecting and the injected rock 

 retain their separate individualities, however thin and frequent may 

 be the sills; whereas at the permeation junctions "the margins of 

 a sill fade off into the gneiss through a series of thinner and thinner 

 lenticles " (PL XIII, Fig. 1), " the ends of a sill also fading off 

 into the gneiss by a dovetailing of biotitic folia into the granite " 

 (PL XIII, Fig. 2). " Finally, large masses occur in which these 

 relations are carried to such a degree of intimacy as to render it 

 very difficult to decide whether to regard them as granite or as 

 gneiss (PL XIII, Fig. 3), difficult even to produce a consistent 

 map, all lines being wholly arbitrary" (op. cit., p. 644).^ 



In the same paper (pp. 642-3) evidence is adduced to show that 

 much of the gneissose rock so permeated must be of sedimentary 

 origin. That it does not consist merely of the material of 

 the adjacent granites altered by marginal shearing is shown by 

 the existence of uninjured intrusive junctions at other parts of the 

 same sill (ibid., figs. 2, 3). In conclusion (ibid., pp. 647-8) it was 

 suggested, though with the caution due to the chemical difficulties 

 to be encountered, that the granites might not be wholly foreign 

 matter ; and this was alluded to in the discussion by several speakers, 

 who pointed out that the suggestion was really a revival of the older 

 theory which I have described above. 



4. Application of Roberts- Austen's Besults. 



Now, in the interpretation of phenomena of this kind the results 

 of Sir W. Eoberts-Austen's experiments seem to open up a prospect 

 of considerable help. In the permeation zones of these granites, 

 whatever may be their cause, we see, at any rate, an unquestionable 

 case of the diffusion of one rock into another. 



Eoberts-Austen has shown (1) that diffusion takes place between 

 closely adpressed solids at ordinary temperature, (2) that with rise 



1 " On Foliated Granites and their relations to the Crystalline Schists in Eastern 

 Sutherland " : Q.J.G.S., 1896. The views of our colleague, the late Hugh 

 Miller, jun., are also given in this paper. 



2 I had not at the time this was written read this passage from Lehmann (Enst. 

 Altkryst. Sch., p. 64) : " Die Abgrenzung zwischen dem, was als Granit oder 

 Granitgneiss und dem, was als Gneissglimmerschiefer zu hezeichnen ware, wird oft 

 unmoglich tmd komnit ganz avf suhjeetives Ermessen hinaiis, so schneU wechselt der 

 Gesammtcharakter," but cannot refrain from quoting it now. I put one phi-ase 

 in italics. 



