﻿520 DR. F. H. HATCH AND MR. R. H. RASTALL ON [Nov. I9IO, 



described ; in others, the position and arrangement of the newly- 

 formed silicates appear to indicate the original presence of layers 

 of impurities along bedding-planes, which have been disturbed and 

 contorted by earth-movements prior to the thermal metamorphism 

 by the granite. 



To conclude : in the Port Shepstone rocks we have a well-marked 

 instance of dedolomitization, in essential features similar to those 

 described by other authors, but differing in its determining factors. 

 So far as we are aware, in no previously described case was the 

 introduction of new material derived from included boulders. 



We have to express our indebtedness to Mr. W. Campbell Smith, 

 of Corpus Christi College, Cambridge, and to Mr. T. Crook, for 

 careful work carried out by them on some of the isolated minerals 

 in the laboratories of the Mineralogical Museum at Cambridge and 

 of the Imperial Institute respectively. 



EXPLANATION OF PLATE XXXV. 



[All the figures, except fig. 6, are magnified 20 diameters.] 



Fig. 1. Olivine-spinel band in the 'dark-mica zone.' = Olivine; S = Spinel; 

 and C = Calcite. Ordinary light. 



2. ' Light-mica zone.' F = Forsterite ; M = Phlogopite ; and C = Calcite ; 



Ordinary light. 



3. Ophicalcite zone. Serpentine (light part) and calcite (dark part). Ordi- 



nary light. 



4. The same slice as the preceding, between crossed nicols. 



5. Reaction-rim of felspar-rock. S = Scapolite in transverse and lon<n- 



tudinal sections. Between crossed nicols. 



6. Spinel-forsterite rock. Spinel (black) ; F— Forsterite ; and C = Calcite. 



Between crossed nicols. Magnified 15 diameters. 



Discussion. 



Prof. Sollas remarked on the extreme interest of the mineral 

 changes described by the Authors, but found a difficulty in under- 

 standing how these changes had been brought about. He had 

 imagined that the word inclusion must have been substituted by 

 an oversight for intrusion, but evidently this was not the case. 

 The assumed transference of silicon dioxide and alumina from the 

 granite to the dolomite required further explanation : it could 

 hardly be supposed that these substances migrated in the free 

 state. Something also in the nature of an exchange was to be 

 expected, affecting the mineral composition of the granite, and 

 giving rise to basic felspars; yet the Authors had observed, 

 evidently not without surprise, that the characteristic felspar was 

 not anorthite but oligoclase. 



Dr. H. J. Johnston-La vis wished to know whether the granite 

 inclusion also showed zoning from chemical interaction with its 

 calcareo-magnesian matrix. The speaker had always maintained 

 the mutual interaction between two such rocks, which in fact 



