GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 469 



In the previous section the contemporaneity of the intrusion 

 of the Prospect mass with the volcanic outbursts of similar 

 rock in the surrounding region was shown to be very prob- 

 able. Some twelve or more volcanic Decks are known 

 within a radius of 40 miles from Sydney. Most of them 

 are filled with agglomerate consisting of decomposed basalt 

 and fragments of sedimentary rocks, chiefly Hawkesbury 

 Sandstone, but four contain solid plugs of basalt in addition 

 to agglomerate. 1 



In the Blue Mountain region, as in New England, a 

 moderate uplift, of about 400 feet in the Blue Mountains, 

 was succeeded by a long period of quiescence (Mr. Andrews' 

 ** Plateau Cycle"); and the great volcanic outburst of 

 basalt appears to have coincided with the earlier part of 

 the slow, but nearly continuous, great uplift of 3,100 feet. 2 

 It is reasonable to suppose that the warping of the Triassic 

 basin took place coincidently with these uplifts ; hence the 

 Triassic strata were probably slightly flexed before the 

 intrusion of the Prospect mass, but considerably more so 

 afterwards. The greatest depth which has been proved in 

 the Wianamatta shales is 800 feet found in a bore at 

 Penrith. 



If we assume that at the time of intrusion the base of 

 the Wianamatta shales was 100 feet lower at Penrith than 

 at Prospect, 3 and that the previous cycle of erosion had 

 produced a fairly level surface between the Blue Mountains 

 and Sydney, not much more undulating than we find at the 

 present day, then there must have been about 100 feet less 



1 M. Morrison, Dykes and Necks of the Sydney District, Rec. Geol. 

 Surv. N.S.W., vii, pt. 4, p. 273. 



2 Andrews, Proc. Linn. Soc. N.S.W., 1903, pp. 803-6. 



3 This difference of level corresponding proportionally to the uplift of 

 the Blue Mountains before and after the great volcanic outburst. We 

 assume the shale to bave been horizontal before Andrews' first uplift 

 (Plateau Cycle). 



