136 H. 8. JEVONS, H. I. JENSEN AND C. A. SUSSMILCH. 



to the increasing proportion of magmatic water and mother 

 liquor introduced as vein formation continued. 



The aplites were clearly formed after the pegmatites, 

 and resulted from the crystallisation of mother liquor only. 

 This mother liquor, owing to its high water content, 

 remained fluid at a much lower temperature than the rest 

 of the magma. After the pegmatites had finished forming, 

 contraction of themow practically solid mass still continued, 

 the two large veins still continued to widen, while a num- 

 ber of smaller cracks, more or less at right angles to them 

 developed. Into all of these fissures mother liquor was 

 squeezed, and from its crystallisation the aplites developed. 

 The progressive increase in the grain size of the aplites 

 was due, no doubt, to a progressive increase in the pro- 

 portion of magmatic water present, the water content at 

 the later stages being increased by the expulsion of mag- 

 matic water by the earlier formed finer grained aplites. 

 That there was a high water content and ample space at 

 the time of crystallisation is shown by the strongly developed 

 miarolitic structure of the coarse aplites. In this last 

 stage of vein formation, pneumatolysis possibly played some 

 part. The reopening of the veins after the fine grained 

 aplite had solidified, fractured and fissured this earlier 

 formed type ; this would account for the patches of fine 

 aplite which occur in places in the coarse aplite, and the 

 thin veins of the latter traversing the former, as shown in 

 figure 2. 



The disposition of the segregation veins at Prospect, 

 their absence in the more compact part of the pallio-essexite, 

 and their absence in the shales, are evidence, in our opinion, 

 that (a) the load of sedimentary rocks overlying the intru- 

 sion was not great, and (b) that the theory of bending or 

 arching of the shales over the rim of the intrusion is prob- 

 ably more correct than that of breaking and faulting, since 



