﻿Vol. 64.] 



ST. DAVID S-ITEAD ' EOCK-SEEIES. 



291 



indeed, seem to be opposed to the theory of diflferentiation in situ. 

 Acid streaks form a characteristic feature in many basic intrusions, 

 as, for example, the coarse pegmatoid veins at Penmaenmav^r, the 

 core of the Whin-Sill intrusion, the coarse veins in the Eowley- 

 Rag mass, etc. In all these cases, however, there is a deficiency in 

 the acid veins of the minerals of early separation. This is not the 

 case in the rocks now under consideration. When we consider the 

 distribution of the felspars, the titaniferous iron-ores, and other 

 facts connected with the sequence of crystallization of these rocks, 

 it is impossible to look upon the quartz-bearing rocks of this series 



Pig. 2. — Diagram illiistratinr/ the chemical relations of the various 

 types of the St. DavicVs-Head '■rock-series! 



as the acid residuum or mother-liquor of a partly- consolidated 

 magma. Perhaps the greatest difficulty in accepting this view, 

 however, is the mode of occurrence of these different types as seen 

 in the field. In many places these are found to pass one into 

 the other by quite sudden transitions ; and in the banded rocks we 

 have evidence that two magmas coexisted side by side, in streaky 

 lines drawn out by flow during their injection. 



I am driven, therefore, to the conclusion that this is a case 

 of intrusion of a mixed magma from a magma-basin in 

 which a partial gravity-differentiation had already 

 taken place. The different degrees of re-admixture, which 

 subsequently occurred, were possibly determined, to a great extent, 

 by local variations in viscosity. 



