GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 459 



to have been the case in the great dolerite (diabase) sills 

 of Tasmania, and others. There was undoubtedly a certain 

 amount of assimilation of the country rock by the magma 

 at Prospect, as will be shown later by petrological evidence. 

 The shales cannot, however, have been melted up in 

 quantity sufficient to make any appreciable difference in 

 the size of the chamber, for the main mass of the rock is 

 distinctly basic (about 42% Si0 2 ) and the shales are rather 

 siliceous containing probably over 65% of silica. 1 The 

 engulfing of fragments torn from the roof by their sinking 

 through the magma is not a way in which the cavity can 

 be enlarged, but by which the latter may rise through over- 

 lying strata, for what is taken from the roof is added to 

 the floor. We do not think such overhead stoping a prob- 

 able feature of the Prospect intrusion. A large fragment 

 of shale, twisted as it was torn from the roof, may be seen 

 in the Reservoir Quarry suspended in the solidified magma 

 only one metre or so below the rock from which it was 

 torn (Plate XXXV). This mass must have been wrenched 

 from the roof before consolidation of the magma on the 

 outer surface began, and already then the specific gravity 

 of the magma had become too great to alJow the shale 

 to sink, for the viscosity of a basic and hydrous magma 

 soon after intrusion cannot have been great enough 

 to prevent sinking. It would be strange then, if many 

 such masses were torn off, that we find no others, either 

 high or low in the reservoir quarry or elsewhere. If on 

 the other hand, engulfing had taken place when the magma 

 was still much too hot for solidification to begin even at 

 the edges, it would have been hot enough to produce more 

 assimilation than we observe. 



1 We have no analysis of the shale in the immediate vicinity of the 

 Prospect mass, but analyses of Wianamatta shale, quoted in Pittman's 

 Mineral Eesources of N.S.W., p. 433, from near Parramatta and from 

 Cook's Eiver, both within eight miles of Prospect, show 85'3 and 71 per 

 cent, respectively. The shale at Homebush contains 65-96%. 



