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



5. Manner and Mechanics of Intrusion. 

 An attempt to elucidate the manner in which the intru- 

 sion took place may not be without interest. The locality 

 is near the centre of the great Permo-Carboniferous basin, 

 and the magma may be presumed to have been forced 

 upwards through about 13,000 feet of marine and coal 

 measures, and about 2,000 feet of Triassic strata, by an 

 earth movement which was probably continuing the warp- 

 ing of the basin, and produced a widespread igneous 

 activity, which will be referred to again. Whether the 

 magma ascended by a fissure or a pipe, soon after leaving 

 the Hawkesbury Sandstone it reached such a level in the 

 shales that the weight of the superincumbent strata 

 became less than the upward lift of the magma. The latter 

 then tended to spread laterally. For some reason, however, 

 it did not follow the horizontal bedding planes, as in an 

 ordinary sill or lacolite, but spread in a conical layer, lift- 

 ing, through a height of 300 feet or more, an oval portion 

 of shale with a rather rounded shallow conical base convex 

 downwards, 700 acres (280 hectares) in area. 



A question difficult to settle is whether the overlying 

 shale was dislocated and faulted up as shown in fig. 1, the 

 corners being rounded off by assimilation or engulfing of 

 of fragments, as indicated by the broken line, or whether 

 the strata were sharply bent as shown in fig. 2. In favour 

 of the hypothesis of bending is the fact that on all sides, 

 wherever the pallio-essexite is visible, it dips outwards 

 gradually. This is particularly noticeable on the northern 

 slope of the mass, and to account for its development by 

 assimilation from a vertical wall, as in fig. 1, would postu- 

 late a greater amount of assimilation than petrographical 

 evidence seems to admit. On the other hand, against the 

 hypothesis of bending is the fact that no exposure of shales 

 with an outward dip has been found. It is true that with 



