VOLCANIC NECK AT THE BASIN, NEPEAN RIVER. 141 



The expression of a change in the nature of host by 

 absorption of xenolithic material is twofold: (a) textural 

 and (b) mineralogical. The first is conditioned by physical, 

 and the second by botli physical and chemical circumstances. 

 Nothing appreciable along the lines of (b) has been effected 

 like the results observed in the Skye rocks, where the 

 chemical compositions of the host and inclusions are anti- 

 thetical. The mild effect of a basaltic magma on basic 

 inclusions as at Dundas and the Basin, and the profound 

 alteration of acid fragments in a similar magma, agree 

 with the view, expressed by Harker, that between magma 

 and xenoliths of like composition, there is a general chemical 

 equilibrium. 1 



It is taken here that the inclusions at the Basin, with the 

 exception of such fragments as quartz, etc., are fragments 

 cognate with the environing basalt, and are the expression 

 of a phase which differentiated under plutonic conditions. 

 Subsequent transport to higher levels has caused rounding 

 of many of the fragments through corrosion and corrasion. 2 

 From microscopic examination it appears that certain 

 processes went on in deeper regions, altering the fragments 

 before they were carried up to their present position. 



The absorption of the magma here, as at Dundas, has 

 been selective, felspar succumbing much more readily than 

 augite. 



The basalt has penetrated intergranular spaces in the 

 xenoliths and especially in those of gabbroic types, showing 

 that the whole of the inclusions w T ere well heated to allow 

 such fine strings of basic material to penetrate so far. 

 Plate VII, fig. 6 shows the manner in which augite is freed 

 by the gradual absorption of felspar. An interesting feature 

 observed was that of a felspar grain adjacent to the magma 



1 A. Harker, "Tertiary Igneous Rooks of Skye," p. 354. 



2 "Origin of Dike Inclusions, Journ. of Geol., 1915, p. 169. 



