part 2] geology or the marble delta. 133 



Along the bank it is 40 or 50 feet wide, and runs up the steep 

 hillside to the top where it dies out ; in it are a number of 

 granite-veins, one of them rich in crystals of sphene 1 cm. long. 

 Connected with these veins are the chondrodite- and spinel- 

 hearing marbles mentioned earlier. The analysis of an average 

 commercial sample showed 1*5 per cent, of magnesium carbonate 

 and 1 per cent, of insoluble matter. 



Such evidence as could be gathered in the field concerning 

 these occurrences pointed, difficult though the explanation may 

 be, to the calcite as being generally, not the mere contact-phase 

 of a non-magnesian limestone-layer within the marble, but the 

 terminal product in the metamorphism of a rather pure variety 

 of dolomitic limestone. 



VII. EXO.MORPUIC AND EXDOMORJL'IIJC ClIAXUES. 



There can be no doubt that originally the marbles were mag- 

 nesian limestone, since converted through thermal (aided perhaps 

 by static) metamorphism into crystalline dolomitic marbles, de- 

 dolomitized in places through the elimination of carbonic acid 

 and its replacement by silicic acid. This normal type of contact- 

 metainorphism, where no additions have been made to the country- 

 rock, does not require further discussion; but in immediate 

 proximity to the contacts appears the ' transf usive facies ' long 

 recognized by the French school of geologists, prominent among 

 whom stands Prof. A. Lacroix. 



In his interesting account of the recrystallized humite-bearing 

 limestone inclusions from Monte Somma, 1 in which a shell of 

 mica, pyroxene, or huniite generally surrounds a kernel of calcite, 

 he has definitely regarded the mineral alterations as the result 

 of volatile emanations from the igneous rock, chiefly chlorides of 

 sodium, magnesium, aluminium, and iron, and also fluorides, fluo- 

 silicates, and silicates of the alkalies. To this list, when the 

 invading rock is granite, should, I think, be added silicon tetra- 

 fluoride, volatile and breaking up in the presence of water vapour 

 into hydrofluoric and silicic acids. 



Various American geologists, among whom may be named 

 .1. F. Kemp and W. Lindgren, have also given instances of what 

 might be called 'additive ' metamorphism accompanying the injec- 

 tion into limestone of igneous bodies : a recent review of certain 

 North American evidence in support of, or against, the two 

 opposing hypotheses has been given by W. L. Uglow. 2 



To sueh emanations must be ascribed the conversion of the 

 marble along the contacts into a nearly pure diopside or a diopside- 

 scapolite-felspar rock, in Natal only on a small scale, as it happens, 

 but well known and by no means insignificant in many other parts 

 of the world. Lacroix's description of the contact-development of 



1 'Les Enclaves des Roches Volcaniques ' Aim. Acad. Macon, vol. x (1893) 

 p. 313. • 



2 'Economic Geology ' vol. viii (1913) p. 19. 



