24 Notes on the Diabase Rocks 



the various works at hand.* I find in them the indica- 

 tive ratio to lie between the following extremes : — 



Diabase (11 analyses) ... 1 -00 — 1-00 to 1-23— 1 00 



Augit porphyr(ll analyses)... 1-00— 1-00 to 1-13— 1 -00 

 Labrador porphyr (6 analyses) 1'24 — TOO to 1-81 — 1*00 



In these calculations there is, however, this difficulty, that 

 there is no means of knowing whether some of the silica is 

 not free, as in the rock I have described ; nor does it seem 

 that in all, or even most, cases regard has been paid to the 

 presence or absence of C0 2 or P 2 5 , nor, the combined H 2 O. 

 The results are, therefore, only approximate, but they may 

 possibly serve to roughly define the limits of the groups. 

 Such being the case, then, this Diabase porphyrite, having 

 the indicative ratio of 1*194 to 1*000, clearly stands at the 

 end of the Diabase series and at the commencement of those 

 porphyritic Diabase rocks which have been called " Labrador 

 porphyr." If, as is probably the case in some of the analyses 

 which I have calculated, all the silica, whether combined 

 or free, were taken into account, then the indicative ratio 

 would rise to 1*384 to 1*000, which would bring this rock 

 well into the " Labrador porphyr" series. Having thus dis- 

 cussed at length the composition and structure of the Diabase 

 porphyrite, I now proceed to show the position it holds in 

 the series of formations amongst which it occurs. 



A mile or thereabouts up the Snowy River from Moore's 

 Crossing, and on the eastern side, there is an excellent 

 example of the contact of the Diabase porphyrite with the 

 Buchan limestones. The details of the various beds, which 

 I now give, commence with the lowest rock visible in the 

 river bed. 



1. Massive Diabase porphyrite forming the river bed. In 

 places it contains amygdules (agate, chalcedony), and also 

 veins and small masses composed of a crystalline aggregate 

 of quartz and epidote. A thin slice examined under the 

 microscope showed the following particulars : — 



The structure is micro-porphyritic. The ground-mass is 

 composed of — (a) a colourless basis full of very numerous 

 minute black dust-like particles, either scattered singly or in 

 groups, or arranged linearly ; (b) minute felspar prisms 

 (triclinic ?); some of tbe minute felspar prisms have sharply 

 defined outlines, while in others the planes are studded with 



* Amongst others, Bischoff, Lehrbuch, &c, 2nd edition, 1864 ; Zirkel, 

 Lehrbuch, &c, 1866 ; Neues Jahrbuchfiir Mineralogie, &c. 



