136 DE. A. L. DL" TOIT O^ THE [vol. lxXV, 



the last-mentioned locality has certainty not changed the adjacent 

 marble. 



The simplest hypothesis is that waters charged with carbonic 

 acid gas ascending along the contacts have removed the magnesium 

 carbonate ; for, although this substance is very much less soluble 

 normally than carbonate of lime, under a pressure of about o atmo- 

 spheres the former is dissolved almost exclusively from dolomitic 

 limestone, according to a statement of Prof. E. W. Skeats. 1 Such 

 waters could have been of magmatic origin, and would then have 

 derived their free carbonic acid through the silication of the 

 calcareous rocks in close proximity. In this connexion there 

 ought to be recollected the intense silication of the marble along 

 the dyke crossing the watershed on Umdwendwe, and also close to 

 the main contact within the western corner of The Glen. The 

 ultimate fate of the magnesia is a matter for speculation, but some 

 of it may not improbably have passed into the invading magma. 



VIII. Summary. 



(1) In addition to the normal type of metamorphism produced 

 by the gneiss and the granitic offshoots therefrom there is also 

 along contacts a phase almost identical with that affecting 

 xenoliths of limestone in volcanic rocks, this similarity having 

 indeed been pointed out by A. Lacroix, C. Dcelter, 3 V. M. Gold- 

 schmidt, and others. 



(2) Through the action of magmatic emanations, zones pos- 

 sessing more or less regularity have been produced in the adjacent 

 dolomitic marbles, of which the innermost is commonly rich in 

 diopside and often in scapolite, with forsterite, phlogopite, 

 chondrodite, and spinel farther away ; dedolomitization is usually 

 perfect. 



(3) In the contact-zone forsterite and chondrodite are anti- 

 pathetic minerals, and the latter is always the farther removed 

 from the intrusion. 



(4) In certain cases the marble beyond the silicate zone has 

 been deprived of the bulk of its magnesia, and has been changed 

 into a mass of coarsely-crystalline calcite. 



(5) This phenomenon, termed calcitization, has probably 

 been due to the action of carbonated waters during the cooling 

 of the plutonic complex. 



(6) The absorption of marble by the magma has led to desili- 

 cation of the latter, and has caused the development of pyroxene 

 in the intrusion ; concentration in alkalies may have been Drought 

 about at the active surface, with the consequent crystallization 

 there of a rock possessing alkaline affinities. 



(7) With regard to the assimilation of marble by the gneiss 

 upon a large scale there is no evidence from this area. 



1 Q. J. G. S. vol. lxi (1905) p. 135. 



2 ' Petrogenesis ' 1906, p. 157. 



[April 17th, 1920.] 



