GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 455 



Wianamatta shales precludes the posibility of using palseon- 

 tological zones to determine the horizon of the central 

 area of shales. Indirect evidence, however, is available. 

 The nearness of the outcrop of the underlying Hawkesbury 

 Sandstone, located about five miles both to east and south, 

 makes it almost certain that the Wianamatta shales 

 which have an extremely small westward and north- 

 westward dip, cannot be here more than at most 300 or 400 

 feet thick, whilst they may be only 200 feet thick. If the 

 intrusive sheet were more than 300 or 400 feet thick it 

 must therefore have lifted Hawkesbury Sandstone ; but 

 there is not the slightest trace of any sandstone beneath 

 the shale of the central area, or even of the shale being of 

 a sandy character. Remembering that the lower surface 

 of the central patch of shale stands about 50 feet above 

 the level of the surrounding country, we thus have 450 feet 

 as the absolute maximum which we can allow for the 

 thickness of the sheet ; but a more probable figure would 

 seem to be about 300 feet. There is nothing in the 

 character of the rock exposed on the floor of the Reservoir 

 Quarry, which penetrates deeper into the mass than any 

 other exposure, which would lead one to suppose that this 

 latter figure is an under estimate. The same quarry provides 

 us with a minimum figure of about 150 feet for the thickness 

 of the sheet, for at its northern end it extends at least 80 

 or 90 feet below the cooling surface, and there is no sign 

 of the grainsize beginning to get finer again at the bottom. 



Whether the intrusion more probably took place through 

 a fissure or a pipe we cannot say, for both these forms of 

 conduit are common in the district. The position of the 

 supply pipe or fissure is also uncertain. It may be under 

 the south-east portion of the hill, because this is the most 

 elevated and massive, or it may more probably, as will be 

 shown later, be beneath the central area of shale. 



