part 2] THE SIIAL' MINOR INTRUSIONS. 1-43 



Fig. 6. An acid dyke near the granite, probably an apophysis showing assimi- 

 lation of country -rock. Quartz is colourless, felspar (both plagio- 

 clase and orthoclase) less clear owing to sericite. The other mineral 

 is diopside moulding apatite and spliene. X about 30 diameters. 

 (See p. 125.) T. 



r The micropbotographs marked M. were taken by the Author, those marked 

 T. by Mr. Tarns, Cambridge.] 



Discussion. 



The President (Dr. A. Harker) congratulated the Author 

 upon a contribution which presented numerous points of interest. 

 While confirming generally the results formerly reached by 

 Prof. Marr and the speaker, this paper seemed to make a decided 

 advance in the interpretation of the mutual relations of this group 

 of intrusions. The roughly- radiate disposition of the dykes about 

 the granite as centre was resolved into two groups, approximately 

 parallel to dip and strike respectively and intruded at distinct 

 epochs. It was in harmony with the received conception of differ- 

 entiation that the extreme types, both acid and basic, should not 

 onlv be found in the outlving members of the svstem. but should 

 also represent the latest injections. Another point of interest was 

 the asymmetric arrangement implied in the situation of the more 

 acid minor intrusions on the northern side of the granite. This 

 connects itself with the unilateral character of the Caledonian 

 ■erustal stresses, and finds a parallel in the horizontal diif'erentiation 

 exhibited by the granite-masses of Skiddaw, Eskdale, and Foxdale. 



Pr. J. W. Evans thought that the facts brought forward by the 

 Author afforded valuable evidence for the solution of the problems 

 •of the mechanism of igneous intrusion. He inquired whether there 

 was evidence of flow-structure indicating the direction of the flow 

 in particular cases. The change of the strike of the intrusions, 

 from one direction in the earlier intrusions to another nearly at 

 right angles to it, indicated an important change in the nature 

 of the forces affecting the district. 



Dr. J. Y. Elsden asked the Author to how great an extent 

 evidences of assimilation could be recognized where intrusions had 

 invaded limestone strata. He thought that the Author had some- 

 what extended the definition of the term ' xenocryst ' in applying 

 it to crystals which had merely changed their position within the 

 magma. This term was more often used to denote undigested 

 inclusions of extraneous matter. It was also questionable whether 

 rocks formed from mixed magmas, differentiated from the same 

 parent, should strictly be called ' hybrid ' — a term which he would 

 prefer to use where no genetic relationship of the mixed types 

 was concerned. Kosenbuscirs petrographic term ' spessartite/ also, 

 appeared to be unfortunate, the same word being in current use 

 in a definite mineralogical sense. These small points of criticism 

 were, however, quite insignificant in view of the importance and 

 value of the Author's communication. 



