part 2] THE GEOLOGY OF THE MARBLE DELTA. 121 



A summary of the general geology has appeared recently in an 

 account of the economic resources of the Marble Delta. 1 



While the tract is one full of problems that have an important 

 bearing upon contact-metamorphic processes, it is unfortunate that 

 time did not permit of more elaborate investigation in the field 

 and in the laboratory. 



Within the region embraced by the accompanying plan (fig. 1) — 

 differing considerably from that given by W. Anderson 2 — the sur- 

 face of the country forms part of a slope falling from 1500 to 

 1000 feet seawards, deeply trenched by the Umzimkulu and 

 LTmzimkuluwana Rivers, in the denudation of which the area has 

 been largely stripped of its cover of horizontal Table-Mountain 

 Sandstone, still preserved towards the west and south. 



The slopes of this picturesque country are steep, and the spur 

 between the rivers is particularly narrow; nevertheless, the fre- 

 quent presence of thick bush and the abundance of fallen blocks 

 render the tracing of boundary-lines often difficult, while there 

 are occasional accumulations of waterwom boulders along the 

 river-banks. There is one conspicuous terrace cut in the gneiss 

 on the left bank of the Umzimkuluwana River, on the farm F.N., 

 about 300 feet above stream-level, over which, as noted by Draper, 

 there lies an abundance of large waterwom boulders, nearly all 

 of Tabic -Mountain Sandstone. 



II. The Maeeles. 



During the course of the survey the main area of the marble 

 was found to cover a tract almost 4- miles long b}^ about 2 broad ; 

 it is not a solid block surrounded by granite and gneiss, as 

 reported, but a bent and twisted mass enveloped b}- the igneous 

 matter, which underlies it at no great distance and forms great 

 inclined or nearly vertical intrusive sheets cutting across it, thus 

 separating it into several distinct portions. On the eastern side 

 are several parallel sills of granite parted by layers of marble. 

 There are also several strips of marble isolated in the gneiss 

 arranged about the borders of the principal mass. 



The marbles are medium to coarse-grained rocks in which the 

 bedding-planes can be recognized by means of lines of various 

 contact -minerals that show up well on the weathered surfaces or 

 through the presence of thin greyish layers. Much of the marble 

 is pure, the percentage of silicates varying from 1 up to 5; there 

 are types very low in carbonate, but these form only a small 

 proportion. The formation must have a thickness of a couple of 

 thousand feet, and is split up into an upper and a lower group 

 by a naiTOW belt of quartz-schist about 50 feet thick; this in- 

 cludes at one point a 15-foot parting of marble. In a few places 

 in this zone there is a little quartz-felspar rock, making it probable 



1 Mem. Geol. Surv. S. Africa, No. 11 (1918) chapt. xiii, with map. 



2 Third & Final Report Geol. Surv. Natal, 1907, pi. xxii. 



