BY W. N. BENSON. 451 



quartz, and this rim consists of minute colourless prisms of diop- 

 side. Around the felspar, no such rim exists ; this mineral appears 

 to have been absorbed into the basalt-magma much more quickly 

 than the quartz-grains, which project out from the general boun- 

 dary of the inclusion into the basalt or are even isolated in it. The 

 single instance of a pyroxene-bearing gneiss shows how much less 

 readily is the pyroxene absorbed than the felspar. It has been 

 shown that this order of solubility holds also in the case of the basic 

 felspar and augite of the gabbroid inclusions of Dundas.* 



The rocks with glass are few in number. In the granodiorite, it 

 occurs in irregular dull-brown patches, more or less cryptocrystal- 

 line with sometimes slag-like skeleton-crystallites, sometimes pene- 

 trated by laths of secondary felspar growing in from the felspar 

 that forms the boundary to the droplet of glass. Frequently, the 

 glass is replaced by chlorite. In another slide, the melt from the 

 gneiss has clearly mingled with the basalt-magma. The zone of 

 mingling is about J inch wide; farthest from the basalt, residual 

 quartz-grains lie in a base originally glassy but now chiefly chlorite 

 and epidote. Nearer the basalt, the glass is filled with felspar- 

 laths in addition to the two decomposition-minerals, and small 

 reaction-rims are seen about the quartz-grains. Nearer still, mag- 

 netite and purple augite-grains occur, and the epidote and chlorite 

 are less abundant; gradually this passes into the normal basalt. 

 One would expect that the felspar varies in composition in the dif- 

 ferent stages, but, unfortunately, a determinative set of readings 

 could not be obtained. The same feature of absorption was seen 

 where there was no glass present. In one slide, one may follow, 

 for the space of about a centimetre, a vein, projecting from the 

 basalt into the gneiss, becoming poorer in coloured constituents as 

 it goes; in another, veins of finely crystallised rock, scarcely a 

 millimetre in width, traversing alkali-felspar grains in the gneiss, 

 have abundant finely divided magnetite in the centre, but are free 

 from it at the sides. In such veins, the felspar-laths make a felt 



* " The Volcanic Necks of Hornsby and Dundas," Proc. Roy. Soc. 

 N. S. Wales, 1910, p. 542. 



