274 DISCUSSION. 
Australian diamonds were harder than Cape or Brazilian diamonds. 
If the statement was correct, the refractive index of the Australian 
diamonds would probably be found to differ from that of Cape 
and Brazilian diamonds. Had the refractive index of Australian 
diamonds been determined? With regard to the important dis- 
covery by the Rev. J. Milne Curran of sapphire in sitw in the 
basalts near Newstead, in the Inverell district of this Colony, he 
wished for further information as to whether (1) More than one 
specimen of sapphire had been found imbedded in the basalt at 
the above locality ; and (2) As to whether the sapphire in the 
basalt showed any evidence of having possibly deen picked up by 
the basalt from alluvial gravels over which the basalt had flowed. 
With regard to the occurrence of turquoise near Bodalla, he would 
like to know whether any trace of phosphatic limestones or phos- 
phatic rocks of any kind had been met in the same locality. As 
to the mode of origin of the precious opal in Australia, while he 
agreed with the suggestion made by the Rev. J. Milne Curran 
that it was possibly diatomaceous, he was inclined to think that 
in view of the latitudes of the opal-bearing localities, and of the 
association, near White Cliffs, Wilcannia, N. 8. Wales, of marine 
shells with the opal rock, that radiolaria had probably played an 
even more important part in supplying material for the opal than 
diatoms. Sections of the opal rock, kindly lent him by the Rev- 
J. Milne Curran to examine for radiolaria showed fairly good 
evidence of the presence of remains of these organisms in the 
form of spherical casts of the interior of the radiolarian shells. 
No definite latticed shell had, however, been as yet detected by 
him. He had observed similar spherical casts in some of the opal 
rock from Queensland. They were just the right size for radio- 
laria, but too large for diatoms. He understood that the Rev. 
J. Milne Curran was quite prepared to admit the possibility or 
even probability that the opal of Australia derived its silica largely 
from radiolaria. He considered that the Society were to be cor- 
gratulated on having presented to them an essay containing such 
important original observations as those made by the author as 
