DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. 137 



the formation of a circular fault would have placed a con- 

 siderable load on the intrusion. A heavy load pressing on 

 the intrusion would have prevented the formation of the 

 two large contraction fissures which parallel the upper 

 margin and would have tended to force the mother liquor 

 into the shales along the fault cracks. The selvage of 

 pallio-essexite formed at the beginning appears to have 

 acted as an impervious screen, which reduced the rate of 

 cooling, hemmed in the magmatic vapours, and gave the 

 whole mass those peculiarities of texture which at first 

 sight suggest origin at a considerable depth. 



3. The Sub-alkaline Nature of the Prospect Intrusion as a 



whole. 



The essexite of Prospect is so essentially similar in chemi- 

 cal and mineral composition to the other Tertiary basic 

 igneous rocks of the Sydney, Blue Mountain and Illawarra 

 Districts, that there can be no question that they have all 

 been derived from a common magma, and that the region 

 in which they occur is a petrographical province, as already 

 pointed out by G. W. Card. 1 It has also been claimed by 

 one of us, 2 that the whole of Eastern Australia had been a 

 single petrographical province throughout Tertiary Time. 



Even if this petrographical province be limited to the 

 central-eastern part of New South Wales, it still contains a 

 great variety of volcanic and hypabyssal intrusive igneous 

 rocks ranging from very acid to very basic and from highly 

 alkaline (15% or more of alkalies) to sub-alkaline or even 

 calcic. 



Any discussion of the origin of the magma which pro- 

 duced the Prospect intrusion necessarily involves, therefore, 

 a discussion of the origin of all the Tertiary igneous rocks 



1 Records Dept. of Mines N.S. Wales, Vol. vu, part 2, page 93. 



2 The Alkaline Petrographical Province of Eastern Australia, by H. I. 

 Jensen, Proc. Linn. Soc. N.S.W., Vol. xxxiii, p. 589. 



