﻿82 



MR. H. G. SMITH ON THE 



[vol. lxxii, 



in consequence of the addition of material from the country rock, 

 fragments having been detached by the magma during intrusion. 

 One has to imagine that the fusion of the derived fragments 

 resulted in the production of areas of abnormal composition, 

 crystallization taking place according to the laws laid down by 

 Prof. J. Morozewics. 1 



As to the amount of assimilation that has taken place, there 

 must be some uncertainty. That this process has been in operation 

 appears to be indicated by the marginal phenomena of the xenoliths ; 

 but the biotite bordering them is not to be distinguished from 

 that of the body of the rock, and the question of the origin of 

 the biotite as a whole at once arises. What proportion of it is 

 due to the absorption of foreign material ? The production of the 

 orthoclase during the later stages of ciystallization indicates a 

 change in the composition of the magma, a change which has 

 to be attributed either to assimilation or to differentiation. 



The facts here put forward are not sufficient to warrant much in 

 the way of generalization, but it would appeal- justifiable to state 

 that a certain amount of assimilation is indicated, and that much 

 more may have taken place before the magma became sufficient!}' 

 viscous to leave any record of the process. 



My thanks are due to Mr. J. H. Williams for the analysis ; to 

 Prof. W. W. Watts for the granting of facilities for conducting 

 the work in the Imperial College of Science & Technology ; and 

 to Dr. H. H. Thomas for valuable suggestions at more than one 

 stage of the investigation. 



EXPLANATION OF PLATES VIII & IX. 

 Plate VIII. 



Fig. 1. The Lurgecombe Mill lamprophyre. X 2S diameters. (See p. 77.) 

 2. Orthoclase, quartz, and rhombohedral carbonate surrounded by biotite. 

 X 34 diameters. (See p. 78.) 



Plate IX. 



Fig. 1. Inclusion in the Lurgecombe Mill lamprophyre. X 25 diameters. 

 (See p. 80.) 



2. Tubular connexions between staurolite and biotite. X 80 diameters. 

 (See p. 81.) 



Discussion. 



The President (Dr. A. Harker) complimented the Author on 

 his paper, and commented upon the various points of interest. 

 Prof. C. Gr. Cullis, after congratulating the Author, referred 



1 ' Experimentelle Untersuchungen iiber die Bildung der Minerale im 

 Magma ' Tschermak's Mitt. vol. xviii (1898) pp. 1-90 & 105-240 ; also T. A. 

 Jaggar, Journ. Geol. vol. vii (1899) pp. 300-13 ; see also Sir Jethro Teall on 

 ' The Natural History of Cordierite & its Associates ' Proc. Geol. Assoc. 

 vol. xvi (1899-1900) pp. 61-74. 



