DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. 135 



The dimensions of the different rock types in the specimen 

 shown on Plate II, as measured from top to bottom about 

 half an inch from the left side of the plate are as follows: — 

 Pegmatite 12 cm., fine aplite 20 cm., coarse aplite 5 cm., 

 fine aplite 42 cm., pegmatite 85 cm. The specimen as shown 

 is not the full width of the vein, some of the pegmatite is 

 missing. 



Origin of the Pegmatites and Aplites. 



Liquation has probably been the main factor in the form- 

 ation of these more acid and alkaline rock types. As the 

 cooling of the whole mass of the intrusion advanced, the 

 more acid and alkaline portion of the mixture showed a 

 tendency to separate from the more femic portions because 

 of the greater miscibility of the former with the magmatic 

 water present, which tended to retain the acid constituents 

 in a fused state ; this acid portion of the magma became 

 a mother liquor. As crystallisation advanced, the mass 

 cooled, and contraction fissures developed. At this 

 period that portion of the magma undergoing crystallisation 

 would be expanding slightly, but those parts of the mass 

 in close proximity to the surrounding shales had already 

 consolidated, and were experiencing the contracting effects 

 of relatively rapid cooling. The contracting portions were 

 rigidly connected with the overlying fused and indurated 

 shales, and as the consolidated portions contracted they 

 were drawn towards the exterior of the mass, thus causing 

 cracks to develop in the zone immediately below. This 

 would account for the parallelism between the main segre- 

 gation veins aud the upper margin of the intrusion. 



Into these contraction fissures, as fast as they formed, 

 would be squeezed a mixture of residual magma (rich in 

 magmatic water) and mother liquor, from this the pegma- 

 tites crystallised. The somewhat higher acidity and coarser 

 grainsize of the coarser variety of pegmatite would be due 



