﻿Vol. 66.~\ DEDOLOMITIZATION IN THE MARBLE OF PORT SHEPSTONE. 507 



21. Dedolomitization in the Marble of Port Shepstone (Natal). 

 By Dr. F. H. Hatch, M.Inst.C.E., E.G.S., and R. H. Eastall, 

 M.A., F.G.S. (Read May 25th, 1910.) 



[Plate XXXV. — Microscope-Sections.] 



Page 



I. Introduction 507 



II. Field-Eelations of the Dolomite 508 



III. Petrographical Description of the Invading Granite ... 509 



IV. Petrographical Description of the Dolomite, its Inclu- 



sions, and its Alteration-Products 510 



(1) The Dolomite. 



(2) The Granite Inclusion and its Eeaction-Border. 



(a) The Granite Inclusion. 



(b) The Dark-Mica-Olivine Zone. 



(c) The Light-Mica-Forsterite Zone. 



(d) The Ophicalcite Zone. 



(3) Other Inclusions in the Dolomite. 



(a) The Felspar-Scapolite-Diopside Eock. 



(b) The Spinel-Forsterite Eock. 



V. Discussion of Eesults 516 



I. Introduction. 



While on a visit to the marble quarries of Port Shepstone, the 

 attention of one of the authors x was attracted by a block of granite, 

 included in the dolomite and surrounded by a dark banded border, 

 which was succeeded in order by a pale brown and a pale green 

 zone. On closer examination these coloured rings were found to 

 consist of a portion of the marble, containing a number of inter- 

 esting minerals in zonal arrangement. Among these minerals a 

 brown mica and a coal-black substance were the most conspicuous 

 to the unaided eye, although a pale mica and a green serpentinous 

 mineral could also be distinguished. On account of its obvious 

 interest a photograph of the occurrence was taken (see fig. 3, 

 p. 512), and a number of specimens were collected for microscopic 

 examination. 



The Port Shepstone marble is well known in Natal, on account 

 of its beautiful white appearance and coarsely crystalline (sac- 

 charoidal) texture. Unfortunately for those interested in its 

 commercial exploitation, it does not possess the uniformly fine and 

 even grain of the celebrated marble of Carrara, a fact which mili- 

 tates against its employment for statuary purposes. It is first 

 mentioned in geological literature by C. L. Griesbach, 2 who notes the 

 occurrence of a ' crystalline limestone of enormous thickness, [but"] 

 whose position relative to the neighbouring strata is not clear.'* In 

 the year 1891, David Draper 3 described it in somewhat more detail, 



[ l Since this paper was read Mr. Eastall has also visited the locality, and 

 his observations confirm those previous]}' made by Dr. Hatch.] 



2 ' On the Geology of Natal ' Quart. Journ. Geol. Soc. vol. xxvii (1871) p. 56. 



3 ' The Marble Beds of Natal ' Ibid, vol. li (1895) p. 51. 



Q. J. G. S. No. 264. 2 n 



