49-i J. Allan Thomson — Diamond Matrices of Australia. 



and if so, it is desirable that a full microscopic description should be 

 published. Under the circumstances, the rock will be here referred 

 to as a dolerite, the term ' diabase ' being discarded because of its 

 ambiguity. 



The following analysis, made by Mr. Mingaye in the New South 

 Wales Greological Survey Laboratory, accompanied the specimens 

 sent by Mr. Pike. In using this analysis it must be remembered 

 that some of the minerals described below in the concentrates may 

 possibly have been included. Thus the presence of Crj O3 is probably 

 explained by the inclusion of chrome-diopside. 



89-78 



Traces of SrO, Zr Oa, and CI. 

 ■■ r; S; NiO, CoO, BaO, and LiO. absent. 



The concentrates consisted of the following minerals, all previously 

 recorded by Mr. Card : — plagioclase felspar, iron-ores, a pale bistre 

 augite, calcite, pyrite, quartz, tourmaline, a pale-green augite and 

 garnet. The first three minerals are doubtless constituents of the 

 rock, while calcite and pyrite may be considered as arising from their 

 alteration. The presence of quartz and tourmaline is, in all probability, 

 to be explained by the inclusion of fragments of the granite through 

 which the granite is intrusive. There remain the green augite and 

 the garnet to account for. 



According to Mr. Pike's account,^ the abundance of small red 

 garnets in the rock is a very striking feature, and he surmises that 

 eclogite inclusions maj- be present. He also quotes Mr. Andre Levy 

 Strauss for the inclusion of contact- altered mica schist, limestone, 

 and shale, but the descriptions given are hardly convincing. 



The principles which guided Messrs. Pike and O'Donnell in searching 

 for the matrix seem well founded, and justify the hope that other 

 matrices may be discovered. They remarked that the diamonds 

 found in neighbouring claims often showed considerable variations in 

 form and colour, and they concluded that this could arise only if the 

 matrix were near. Waterworn boulders of a decomposed yellow 

 rock were the only constituents of the gravels that seemed likely to 

 be the home of diamonds (arguing from analogies with the yellow 

 ground of Kimberley), and these led them up to the dolerite. 

 Mr. Pike appears still convinced that his matrix is comparable to 

 the blue ground of Kimberley. 



Another supposed matrix of the diamond occurs at Euby Hill, about 



1 A. E. Pike, " Tlie Copeton Diamonds, New South Wales. In Matrix and 

 Notes on the Alluvial Deposits " : The Australian Mining -Standard, January 27, 

 p. 93, and February 3, p. 119, 1909. 



