GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 461 



block of shale. The only condition which would seem to 

 permit floating up would be the formation of a conical 

 fracture by the forcing up of the magma, so that any 

 raising of the block of shales would widen the fissure. 

 Calculation shows that taking the density of the liquid 

 magma as high as 2*9 (solid 3'06), and that of the cold shale 

 as low as 2*5, and assuming the thickness of the sheet at 

 that minimum possible, viz. 150 feet, the top of the intru- 

 sive sheet must have been more than 900 feet vertically 

 below the level of the surrounding country." 1 



Independent evidence will be quoted in Section 6 to show 

 that the depth of the sheet of magma is unlikely to have 

 been more than 600 feet, or at the very most 800 feet, hence 

 the hypothesis that the cavity of the Prospect mass was 

 formed by floating up a block of shales appears unlikely. 



Our conclusions as to the solid form of the Prospect mass 

 may be taken as summarised by fig. 3, if this section be 

 regarded as crossing the mass along its shorter axis, i.e., 

 E.N.B. and W.S.W. (magnetic). On account of the oval 

 shape of the mass a section through its major axis would 

 be longer, and the dip of the sheet less. In form the mass 

 is not unlike a shallow oval funnel, if we assume a central 

 supply pipe ; and if we suppose the funnel made of enamelled 



1 Let x be the depth of the top of the sheet below the undisturbed 



ground level, then x-\- 150 is the thickness of the upraised block, and, 



assuming the fissures to be filled to the ground surface, we have : 



x + 150 2-9 , fto w. c 



—J = — —and x — 937*5 



x 2-6 



The value of x would be diminished if the specific gravity of the liquid 



magma were taken higher and that of the shale lower, but the maximum 



and minimum figures respectively have been used which appear to us 



conceivable for these specific gravities. Very probable figures are 2*55 



and 2 "8 for the shale and magma respectively, which give x = 1,530 feet. 



Our knowledge of the specific gravities of magmas, that is, of the volume 



change on fusion, and with rise of temperature, is still too uncertain 



(Harker, Nat. Hist. Igneous Rocks, pp. 158-9) to admit of more accurate 



calculation. 



D 1— Dec. 6, 1911. 



