﻿Vol.66.] DEDOLOMniZATION IN THE MAEBLE OF PORT SHEPSTONE. 509 



of the examination made by one of us, the marble was seen cropping 

 out on the farm JSPclongeni, about a mile north-west of the nearest 

 point on Anderson's boundary ; and an isolated outcrop was observed 

 in Cherry willingham Park, at a point a mile and a quarter north- 

 east of his limit in that direction. 



During the same visit a traverse was made along the right bank 

 of the Umzimkulu Biver, and the marble was there seen to be cut 

 by several big dykes of red granite, striking roughly east and west. 

 The width of one of these was roughly measured by pacing, and 

 found to be from 200 to 300 feet. The granite shown in Draper's 

 section of Indwendwa Hill is evidently a portion of the outcrop of 

 one of these big dykes. Whether they are to be considered as 

 offshoots from the main mass, or as later intrusions, cannot be now 

 decided. Their appearance is rather different from the general 

 character of the main mass of granite, which has a prevailing grey 

 tint. In any case, they must have had considerable influence on 

 the metamorphism of the dolomite. 1 



The invading granite surrounds the dolomite on all sides, and 

 covers a large area of country extending for many miles to the 

 north. To the west of the dolomite area it is overlain uncon- 

 formably by horizontally -bedded Table Mountain or Waterberg 

 Sandstone, 2 and to the east and south-east it is covered by Dwyka 

 Conglomerate and Ecca Shales. Since the Table Mountain Sand- 

 stone at the Cape underlies beds of Devonian age, the intrusion 

 of the granite was certainly pre-Devonian. The dolomite and 

 certain schists which accompany it presumably belong to the 

 Swaziland System. 



III. Peteographical Description of the Invading Granite. 



Specimens of the invading granite were collected from a good 

 exposure 2 miles north-east Of the Marble Delta Company's Quarry, 

 near Sinclair's house on the left side of the Umzimkulu River. It is 

 a rather dark-grey rock of medium texture, and consists exclusively 

 of quartz, alkali-felspar, and brown biotite. Under the microscope 

 the most abundant constituent is seen to be microcline, which, 

 together with quartz, makes up the bulk of the rock. A little 

 orthoclase is present, but no plagioclase can be identified. Some 

 of the microcline crystals show indistinct perthitic structure. The 

 mica when fresh is bright yellowish brown and strongly pleochroic, 

 but in places it is partly altered to chlorite. The structure is 

 typically granitic, the microcline and the quartz having crystallized 

 almost simultaneously. In places there is even a tendency to 

 graphic intergrowth. 



1 A specimen of one of the dykes of red granite was collected, but no thin 

 section has been prepared, as this specimen was included in the collection 

 presented by Dr. Hatch to the Pietermaritzburg Museum. 



2 This Natal sandstone formation constitutes an important link between the 

 Table Mountain Sandstone of the Cape and the Waterberg Sandstone of the 

 Transvaal. Its correlation with the Cape formation is generally admitted ; 

 but the resemblance to the Waterberg formation, especially in regard to its 

 highly developed basal conglomerate, is even more striking. 



2n2 



