BY ERNEST W. SKEATS, D.SC, A.R.C.S., F.G.S. 



19 



Annual Report of the Department of Mines, New South 

 Wales. 1004, pp. 145-140. 



7^ 



7ZZ2 



Lower Basalt. 



t'l'PER Basalt. 



( 'oMMON Black Opal. 



SKETCH .SECTION sluiwing ocounence of Common Black Opal 

 bt'twoen the two flows of Basalt at Point Danger. Vertical and Horizontal 

 Scale 200 feet to one inch. 



Mr. Andrews described the basalt as of Tertiary age, 

 resting on a denuded surface of (?) (Vxrboniferous rocks 

 nearly at sea level.. As basalts of Jura-Trias age are known 

 from adjoining areas, the precise age of the Tweed Heads 

 basalt is, perhaps, to some extent, an open question. Mr. 

 Andrews' description and section show only one flow of 

 basalt. Mr. Richards, however, who is working at the 

 geology of that area, tells me (see section) that two flows 

 occur ; that certain depressions probably formed by 

 erosion occur on the surface of the older flow, and that 

 the occurrence of the black material a])pears to be restricted 

 to these holloA\'s. Tlie later flow sealed them up and they 

 have been recently exposed during quarrying operations. 

 The quarries occur near Point Danger and about 100 yards 

 south of the Queensland ])ortler. 



Identification of the Material, 

 .Specific Gravity. Two d exterminations with a 

 Walker's steel yard gave values of 2.07 and 2.09. The 

 specific gravity of the tachylyte from the Merri Creek, 

 near Melbourne, is 2.74, the normal value for a glassy basic 

 rock. Th? specific gravity of opal ranges from 1.9 to 2.3 



Microscopic Characters. 

 Five rock sections have been prepared, one of which 



