GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 511 



occasionally used. At first sight there appears to be every 

 justification for the continued use of these terms, for the 

 rock undoubtedly has essentially the mineral composition 

 and the habit of a dolerite (diabase), and also the chemical 

 composition, as was shown in the foregoing section. But 

 when account is taken of the presence of biotite, and of 

 the occurrence in the mass of segregation veins containing 

 as much as 18 per cent, of orthoclase, and a little segyrine- 

 augite, the similarity to normal dolerites is felt to be less 

 marked. In addition to this there is the fact that, as 

 explained in the first part of this paper, the Prospect rock 

 appears to be genetically associated with indubitably alka- 

 line rocks. Where it has cooled rapidly it is indistinguishable 

 under the microscope from basalts of the district, which 

 pass by almost imperceptible gradations into nepheline 

 basalts, such as those of Lue and Burragorang. It would 

 seem that the Prospect rock and the various basalts must 

 be regarded as the hypabyssal and superficial representa- 

 tives of the basic magma which resulted from deepseated 

 differentiation of a primordial alkaline magma. The more 

 acid products of this differentiation were intruded at the 

 same time as the basalts, possibly in early Tertiary time, 

 but for some unknown reason, have only appeared at the 

 present surface around the periphery of the great Sydney 

 basin, as at Jamberoo, 1 Cambewarra and Mittagong in the 

 south and south-west, at Barrigan and Orange in the west, 

 and the Nandewars in the north. If, then, we are right in 

 regarding the magma of the Prospect rock as the basic 

 differentiation product of an alkali mother magma, there is 

 good reason to apply to it the name usually applied to the 

 non-lenadic intrusive rocks of such magma, namely essexite. 



1 Eec. Geol. Surv. N.S.W., Vol. vnt, p. 1. 



