100 On tlie Occurrence of tlie Diamond near Mudgee. 



Silurian coral Favosites GotMandica, beautifully preserved in 

 silica ; also, one slate boulder full of a small OrtJiis. Fragments 

 of brown ferruginous wood have also been detected in the 

 cement. 



It is worthy of remark that the Older Pliocene drift is 

 remarkably free from any detritus of the rocks of the immediate 

 neighbourhood. 



In the waste heaps round the mouths of the shafts in the 

 neighbourhood of basalt and greenstone some curious natural 

 changes can be observed going on : botryoidal masses of hydrated 

 mixed carbonate of lime and magnesia gradually form and bind 

 the loose material together. A hard mineral, not unlike opal in 

 appearance, is also produced in a similar manner, encrusting 

 gravel, timber, old tools, or any material with which it comes in 

 contact. It is a pure hydrated carbonate of magnesia, containing : 



Magnesia 46-99 



Carbonic acid 4978 



Water 4-08 



100-85 

 Specific gravity, 2-94 



"We have now to add a list of the gem-stones and heavy 

 minerals which exist in the drift, and accumulate in the processes 

 of washing for the diamond : — 



1. Blach vesicular JPleonast. — This mineral occurs in small 

 grains from l-20th to ^ inch, and is by far the most abundant. 

 It has a dull black surface, but shows a brilliant fracture. Some 

 pieces are coated bluish-grey, or ferruginous brown; but the 

 interior is the same in all, and the differences would seem to be 

 the result of surface decomposition. It never occurs in crystals, 

 nor shows any ti'ace of faces ; it has no cleavage ; its fracture is 

 conchoidal and jet black, with a strong vitreous lustre. Hardness, 

 8 ; streak, grey ; composition, found by analysis — 



Silica (and undecomposed) 2'75 



Alumina 6429 



Chromic oxide 462 



Magnesia 21-95 



Ferreous oxide 4-49 



98-10 

 Oxygen ratio, 3-2:1 

 Specific gravity, 3-77 



The mineral is amorphous and vesicular. The latter character 

 is remarkable ; the grains do not all show it in the same degree ; 

 one variety with a lustrous surface shows it best, the grains 

 resembling a perfect cinder when seen through a lens. Several 



