GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 463 



A somewhat more elaborate explanation is ventured by 

 us, though merely as an hypothesis. Since the intrusion is 

 most probably of Tertiary age, as will be shown later, it 

 may be assumed that the drainage of this portion of the 

 country was practically the same as it is now, even in 

 detail, where undisturbed by local causes. It is possible, 

 then, that the magma in making its way upward through a 

 pipe or fissure, happened to issue from the sandstone beneath 

 a minor, or tributary, valley. Now the hydrostatic pres- 

 sure exerted by the magma would suffice to lift a given 

 weight per square foot, and thus a given vertical thickness 

 of shale, but no more. The magma would start to lift the 

 shales immediately over the pipe, and would then begin to 

 spread out. It could not, however, spread horizontally; 

 for, as the surface of the ground rose on either side, that 

 would involve lifting a greater thickness of shale than was 

 possible. It would be able to spread only along a surface 

 always at uniform depth below the surface of the ground. 

 Thus the under surface of the block of shale lifted would 

 conform exactly to its upper surface. Were this so, we 

 should expect the shape of the mass not to be that of a 

 dish with a rim of uniform height all round, but to show 

 two depressions in its rim where it was crossed by the 

 overlying valley bottom. Some such depressions do indeed 

 exist — one to the south running into Booth's Quarry, the 

 other to the north, occupied by the existing valley which 

 drains the central area of the hill (See Plate XXXIV). That 

 this latter was a real depression in the ridge of the intru- 

 sive mass, and has not merely been eroded, appears from 

 the trend of the layer of pallio-essexite, and by the dis- 

 position of the shale, which was almost certainly continuous 

 through this valley in geologically very recent times. The 

 other irregularities of the surface of the igneous mass, to 

 the N.W. of Booth's Quarry, are not easily explained, 

 though they might represent minor channels or drainings 



