468 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



The Post-Triassic acid and intermediate flows and intrus- 

 ions on the periphery of the Triassic basin may perhaps 

 be hypothetically assigned to some part of the same period 

 of activity; and the same may be said with more confidence 

 of the basic intrusions of the centre of the basin. The 

 basaltic rock composing the dykes and necks between 

 Sydney and the Blue Mountains, and also the compact outer 

 envelope of the Prospect Mass, resemble the basalts of the 

 Blue Mountain and Moss Vale districts so closely that they 

 are frequently quite indistinguishable one from another in 

 thin sections under the microscope. The color, habit and 

 proportions of the minerals, and the rock-fabric, are exactly 

 the same, hence there seems to be little doubt that the 

 intrusion of the Prospect essexite may be referred to the 

 same period as the extrusion of the great lava flows to the 

 W. and S.W. 



The age of the great basalt outpourings is fortunately 

 determined approximately by the old river gravels and clays 

 which the"y covered. In the deep leads of Gulgong, near 

 Mudgee, and beneath the basic flows of Wingello near 

 Moss Vale, have been found deposits containing leaves of 

 Cinnamomum and other plants of Upper Tertiary character. 

 The vast basalt flows of New England are assigned by Mr. 

 Andrews to the Middle Tertiary. 1 We shall probably then 

 not be far wrong in assigning to the Prospect intrusions a 

 Tertiary Age. 



7. Depth of the Intrusion. 



The grainsize of the Prospect essexite, and the thickness 

 of the compact and fine-grained envelope, as well as the 

 nearly idiomorphic shape of the felspars, all point to the 

 hypabyssal character of the intrusion. It is possible, how- 

 ever, to obtain some more definite idea of the depth beneath 

 the then existing surface at which the mass consolidated. 



1 Loc. cit., p. 197. 



