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



pea to that of a large compound potato, which latter they 

 resemble in shape. Finding however, that the red soil 

 could be traced in some directions right down to the bed of 

 the creek, draining the valley on that side, we were at a 

 loss to know how mucli to allow for surface creep, and came 

 to regard the change of slope already mentioned as more 

 trustworthy, except on the S.W. of the hill where the 

 proximity of the Prospect Greek has led to considerable 

 denudation. 



The original spacial form of the intrusive mass may be 

 inferred from the map and section. The mass was roughly 

 oval in its horizontal extension. Its upper surface was 

 depressed in the centre, and rose to a ridge all round, or 

 nearly all round. Outside the ridge its surface probably 

 fell everywhere steeply outwards. The under surface is 

 nowhere exposed, and its shape is wholly a matter of con- 

 jecture. That the intrusive mass has roughly the form of 

 a sheet seems probable, partly because of the well known 

 tendency of basic magmas to lift strata bodily and spread 

 out in horizontal sheets, but still more because the shales 

 of the overlying central patch are almost everywhere 

 practically horizontal, just as are the undisturbed shales 

 throughout the district. Only in one place has a dip of as 

 much as 10° been observed (in Booth's Quarry), and here 

 the dip is nearly parallel to the surface of the mass. It is 

 impossible that the cavity formed by the mere lifting of a 

 block of strata, without bending or tilting, should have any 

 other shape than that of a sheet of uniform thickness, 

 though if the under surface of the block be not flat, it 

 necessarily cannot be a flat sheet. The best conclusion 

 from the available evidence seems to be that the Prospect 

 mass was originally a sheet, shaped like a round bottomed 

 oval dish with the convexity downwards. 



As regards the thickness of the sheet there is no direct 

 evidence ; and the unfortunate scarcity of fossils in the 



