128 H. S. JEVONS, H. I. JENSEN AND C. A. SUSSMILCH. 



present in the original magma, as is the case, their growth 

 must have been fed with material by convection currents 

 from other parts of the intrusion, as described by H. Si 

 Jevons in Part I. Harker 1 is of opinion that convection 

 can play at most a minor part in fractional crystallisation* 

 and considers diffusion to be the means by which the degree 

 of supersaturation for crystallisation is maintained. The 

 other differences in mineral composition require a different 

 explanation. The pallio-essexite, as shown by its position 

 and finer grain size, must have solidified while the bulk of 

 the intrusion was still molten, and probably acted as a slow 

 conducting layer which retarded the cooling and solidifi- 

 cation of the intrusion as a whole. The main body of the 

 essexite solidified much more slowly, and was, as will be 

 described presently, subjected to other processes of differ- 

 entiation which produced firstly the felspathic phase of the 

 essexite, and secondly the differentiation veins (essexo-aplite 

 and essexo-pegmatite); the already solidified pallio-essexite 

 therefore escaped these later differentiation processes, and, 

 except for its higher proportion of olivine (and perhaps 

 biotite) produced by fractional differentiation as already 

 described, it may be taken as representing the original 

 magma of the intrusion. This would explain its lower con- 

 tent of augite and iron ores and its more acid felspar. 



Absorption of the overlying shales is a possible factor 

 which must not be overlooked. These shales are of a 

 silicious type, and have been somewhat indurated for a 

 distance of about twelve inches from the junction; in places 

 they appear to have been actually fused by the heat of the 

 intrusion. This question of chemical assimilation of the 

 shales has already been fully discussed in our previous 

 paper, 2 and the conclusion arrived at was that only very 

 slight assimilation had taken place. 



1 The Natural History of Igneous Bocks, Alfred Harker, m.a., f.r.s., 

 p. 319. 



2 Geology and Petrography of the Prospect Intrusion, this Journal, 

 1911, page 521. 



