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



iron, the iron may represent the coarse rock, and the enamel 

 the compact outer envelope or pallium. An appropriate 

 name for a form of intrusion so unusual is by no means 

 obvious. It is not a thick sill, as it does not follow the 

 bedding planes; it can hardly be called a laccolite without 

 considerably stretching the definition of that term, though 

 in its mode of formation it does bear a considerable affinity 

 to intrusions of that class; and probably it is not nearly 

 thick enough to be called a boss. Hence none of the 

 recognised names are applicable. It is not usual to include 

 the supply pipe in the conception of the form of a mass; and 

 the likeness of the form of the Prospect mass to a round- 

 bottomed oval dish may perhaps justify its being termed a 

 dish-shaped sheet. 



As has been stated already, the magma lifted a block of 

 shale with a bottom convex downwards, 1 but the reason 

 for this peculiarity of shape has not yet been approached. 

 Two explanations may be suggested. Professor T. W. E. 

 David has explained to us in conversation that he regards 

 a shallow cone as a very likely shape for a fissure caused 

 by a magma rising very rapidly — almost with explosive 

 violence. He quotes in illustration of this view the fact 

 that when a heavy piece of roofing slate is smartly struck 

 near the centre by a blunt iron point an obtusely conical 

 flake of slate is pushed out on the other side. The magma, 

 rising from the Hawkesbury Sandstone with sudden violence, 

 might quite conceivably strike the overlying shale with 

 such force as to weaken it along the surface of a shallow 

 cone, which would then naturally provide the easiest path 

 along which the magma could spread. The oval shape of 

 the cone could be explained by assuming that the magma 

 rose from a widened fissure. 



1 See above and the section in figs. 1 and 2, also Plate XXXIV. 



