DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. Ill 



THE DIFFERENTIATION PHENOMENA OF THE 

 PROSPECT INTRUSION. 



By H. Stanley Jevons, m.a., b.Sc , H. I. Jensen, d.Sc, and 



C. A. SUSSMILCH, F.G.S. 

 With Plate II. 



[Read before the Royal Society of N. S. Wales, July 3, 1912.~\ 



Introduction. 

 In the 1911 volume of these proceedings was published an 

 account of the geology and petrology of the Prospect 

 Intrusion. 1 This intrusion was shown to consist of an 

 essexite (dolerite) occurring in the form of a saucer-shaped 

 sill. It was also shown that the Prospect mass contains a 

 considerable number of rock types differing in composition 

 but clearly related to one another. Nowhere have we 

 found any evidence whatever, that, after the consolidation 

 of the main mass it was intruded by any foreign magma. 

 The only phenomenon which might seem to require this 

 explanation is the occurrence of what we have called 

 pegmatitic and aplitic veins; but such an hypothesis seems 

 to be rendered untenable by the following facts: — (1) that 

 these veins have never been found penetrating the surround- 

 ing shales, (2) that they have no regular fine-grained 

 selvage due to rapid cooling, (3) that, with the exception 

 of aegyrine-augite, they consist entirely of minerals present 

 in the main mass, though there is an augmentation of the 

 felspathic constituents, and (4) that there appears to be in 

 many places a gradual transition through rocks of inter- 

 mediate composition from the main mass to the aplitic and 

 pegmatitic types. 



1 The Geology and Petrography of the Prospect Intrusion by H. S. Jevons, 

 H. I. Jensen, T. G. Taylor, and C. A. Sussmilch, this Journal, XLV,p.445. 



