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



towards the still advancing edge, the thickness of over- 

 lying shale was diminished ; and the pressure of the 

 magma could, therefore, cause it to undergo an increased 

 elastic deformation, which allowed the sheet to become 

 thicker. Still further advance would cause the bending 

 pressure to be beyond the elastic limit of the now thinner 

 covering of shale. The shale would fracture or shear in 

 innumerable places, producing a monoclinal fold ; and the 

 magma would then advance no further horizontally, but 

 begin to lift the whole covering of shales by thickening the 

 sheet. If the pressure of the magma was not sufficiently 

 relieved by the folding of the shales, they probably con- 

 tinued to rise by the formation of a clean fracture, or fault, 

 producing the structure shown diagrammatically in fig. 3. 

 The point of this explanation is that the folding of the shales 

 which was shown above to be the most probable explanation 

 of the peculiar form of the intrusion, is accounted for by 

 the magma passing outwards on two sides at least beyond 

 the divides of neighbouring valleys. The ridges of the 

 intrusive mass would thus lie nearly under these divides ; 

 and the main watershed between the Parramatta and 

 George's Rivers may have passed under Clump Hill. The 

 south side would be the only one on which the surface of 

 the ground had not a rising, but a falling, gradient immedi- 

 ately the magma sheet began to expand from its centre ; 

 hence in this direction the magma would travel horizontally 

 along the bedding planes of the shales, and reach the depth 

 shallow enough for it to fold the shales sooner than in other 

 directions. It would thus tend to swell up here before the 

 folding was produced in other parts, and give the over- 

 lying shale at this point an inward dip. This view affords 

 an explanation of two important facts: that the covering 

 of shale is observed to have an inward dip of 10° at Booth's 

 quarry (the probable point where the old valley crossed 

 the edge of the intrusion), whilst at the Reservoir Quarry 



