666 Geological Society : — 



Shepstone (Natal) — rocks that have already been the subject of 

 several communications to the Society ; but its main object is to 

 demonstrate that certain ' boulders ' of alkali- granite, formerly 

 regarded as inclusions, are in reality parts of intrusive tongues, 

 and to discuss the mutual relations of the igneous rocks and 

 the adjacent dolomites. 



The main area of Marbles covers a tract of about 8 square miles. 

 It is not a solid block surrounded by granite and gneiss, but a bent 

 and twisted mass enveloped and underlain by igneous material, and 

 cut into several distinct portions by great intrusive sheets. 



The Marbles, almost wholly dolomitic in composition, are medium 

 to coarse-grained rocks that have their bedding-planes marked out 

 by various contact-minerals. They reach a total thickness of about 

 2000 feet, and are divided into an upper and a lower portion by a 

 narrow belt of quartz-schist. 



The plutonic rocks are, for the greater part, coarse-grained 

 biotite- or hornblende -orthogneisses, with streaks and belts of 

 hornblendic gneisses, schists, and granulites at or near the contacts 

 with the Marbles. A conspicuous feature of the igneous rocks is 

 that, for a distance of 2 to 8 miles from the contact, they contain 

 red and brown garnets. 



In addition to the normal type of metamorphism produced by 

 the gneiss and the granitic offshoots therefrom, there is also 

 developed at the contacts a phase almost identical with that pre- 

 sented by xenoliths of limestone in volcanic rocks. Through the 

 action of magmatic emanations, zones possessing more or less 

 regularity have been produced in the adjacent dolomitic marble, 

 of which the innermost is commonly rich in diopside and often 

 in scapolite, with forsterite, phlogopite, chondroclite, and spinel 

 farther a way. The dedolomitization is usually perfect. 



In the contact-zone forsterite and chondroclite are antipathetic 

 minerals, the latter being invariably farther removed from the 

 intrusion. 



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. This phenomenon has probably 

 been due to the action of carbonated waters during the cooling of 

 the plutonic masses. 



The absorption of marble by the magma at the contacts has 

 caused the development of pyroxene in the intrusive rock, but there 

 is no evidence in this area of large-scale assimilation of marble by 

 the gneiss. 



