208 E. Greenly — Diffusion oj Granite into Schists. 



Kods of the metals were placed end to end, and in each case the 

 gold diffused upwards into the lead. 



At the end of four years, at only 18° C. (ordinary Summer 

 temperature), gold could be detected 9'95 mm. from the contact. 

 At 251° C, which is still 75^ C. below the fusion-point of lead, at the 

 end of 31 days, '002 per cent, of gold was found 7 cm. from the 

 contact. If the column of lead was kept liquid, the diffusion wa& 

 much faster. But as much gold would pass up into liquid lead iii 

 a day as into solid lead at 18° C. in 1000 years. 



It is clear, therefore, that in solids, as General McMahon remarks, 

 as well as in liquids and gases, there is a good deal of molecular 

 movement ; and as we cannot suppose this to be confined to a few 

 cases only, we may expect diffusion to take place between many 

 solids under favourable conditions. Any pair of solids cannot, of 

 course, be expected to diffuse, any more than any pair of liquids — 

 mercury and water, for example. But solids with as much in 

 common as most silicate-bearing rocks might reasonably be expected 

 to do so. 



2. The MetamorpMc Theory of Igneous RocTcs. 



Before attempting to apply Eoberts-Austen's results, it will be 

 desirable to refer to the relation of granites to crystalline schists 

 in highly metamorphic regions ; and, first of all, to review a theory 

 which at one time had much influence upon geological opinion, and 

 even now continues to recur from time to time to the mind of 

 the worker in regions of this description. 



The eruptive nature of gi-anite has, ever since the classic 

 demonstration of Hutton, been rightly regarded as one of the 

 established truths of geology, and this has been confirmed by 

 numberless examples since discovered in all parts of the world, and 

 in rocks of all ages. 



But the phenomena to be seen at the margins of granites do not 

 always show clear evidence of intrusion, and the study of some of 

 these led to a modification, about the middle of the nineteenth 

 century, of Hutton's original view. That granites are often, perhaps 

 generally, intrusive, was never, I believe, denied. But it wa& 

 asserted that in many cases the margins showed a gradual transition 

 into the material of the surrounding rocks ; and it was inferred that 

 these rocks had been in such cases, not merely altered in mineral 

 character, but actually melted down, and had recrystallized in 

 cooling as granitoid material ; that, in fact, the granite was, in 

 part at any rate, of metamorphic origin. From this view it was 

 an easy transition to that according to which such granites were 

 regarded as of metamorphic origin throughout their whole body ; 

 the heat to which such fusion was due being then ascribed, not to 

 intrusion of heated foreign matter, but to local intensification of the 

 internal heat of the earth. A comprehensive resume of the theory 

 is given, with his usual admirable lucidity, by the late Professor 

 A. H. Green in his "Physical Geology" (ed. 1882, pp. 399-455). 



Unfortunately, however, the theory was not always applied in 

 this moderate and scientific spirit. The chemical composition of 



