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



of Wiauamatta shale at Prospect than at Penrith. An 

 estimate of the thickness of shale removed in the neigh- 

 bourhood of Penrith since the intrusion is now necessary 

 to give us the thickness of shale at Prospect. Penrith lies 

 nearly over the synclinal axis of the elongated basin, and 

 the ground in that district must have been gradually sink- 

 ing, or at least have remained stationary, whilst the country 

 to the east and west was rising. It is very unlikely, there- 

 fore, that denudation of the shales had been rapid here 

 since the time of the intrusion, and the removal of the 200 

 feet of shale seems to be the utmost that can be allowed. 

 If this be granted, the thickness of the shale at the time of 

 the intrusion cannot have been more than 1,000 feet at 

 Penrith and 900 feet at Prospect. 



If we next assume that the Wianamatta shales are now 

 only 200 feet thick at Prospect — a very moderate estimate 

 — and allow 100 feet for the height which the top of the 

 intrusion must have stood above what is now the surface 

 of the surrounding shales, we have remaining 600 feet as 

 the depth at which the top of the intrusion must have, lain 

 below the surface level of the surrounding country. Taking 

 the thickness of the intrusive sheet, as 300 feet as was 

 before seen to be probable, and remembering that the shales 

 must have been lifted to this amount over the sheet, we 

 have the figure 900 feet as probably the approximate thick- 

 ness of the shales which covered the intrusion. 



It is more satisfactory to try and fix limits to the depth 

 of the original shale-covering than to advance a single 

 figure. It will be noticed that the assumption of greater 

 denudation at Penrith increases the figure, whilst the 

 assumption of more warping before the intrusion reduces 

 it. To obtain the maximum, assume that 300 feet of shale 

 have been removed at Penrith, which is hardly conceivable, 

 and that there was no warping at all prior to the intrusion, 



