1872.] DAINTEEE GEOLOGY OF QTJEEJfSLAND. 315 



alkalies present doubtless represent a proportionate percentage of 

 felspar, though a section for the microscope has not been prepared 

 sufficiently thin to obtain information on the latter point. 



The analysis of the prehnite rock afforded me : — 



SiUca 42-033 



Alumina 21-606 



Ferric oxide 8-829 



Lime 23-633 



Water, constitution | n.nAA 



"Water, hygroscopic J 



Copper carbonate 0-825 



Specific gravity 2-844. 99-826 



After examination under the microscope, Mr. AUport says of this 

 rock : — 



" It must have been originally as scoriaceous as any recent lavas, 

 but now forms a hard solid mass of zeolitic mineral matter. 



" All the cavities are filled with prehnite, in radiating groups of 

 crystals, which exhibit a magnificent display of colours when ex- 

 amined in polarized light. 



" One cavity is lined with the radiating prehnite, and the central 

 portion filled partly with calcite, and partly with green carbonate of 

 copper. 



" Of the original constituents nothing is left except the magne- 

 tite. The forms of felspar crystals are sharp and distinct ; but the 

 original substance has been removed, and replaced by prehnite, dif- 

 fering in no respect from that filling the cavities. 



" The reddish-brown grains and patches scattered through the 

 base probably represent the augite." 



It is interesting to observe how nearly these cupriferous altered 

 dolerites resemble in composition the interstratified cupriferous traps 

 of the Potsdam-sandstone series in the Lake-Superior district in 

 America. 



Near the Dotswood Station, on a tributary of the Burdekin river, 

 metallic copper, imbedded in a quartz matrix, has been observed in 

 thin irregular patches. 



The rock enclosing this is amygdaloidal dolerite, itself containing 

 a little copper and copper-ore, but only near the patches of quartz. 



Near the Mount Coora almost the same conditions of cupriferous 

 impregnation of decomposed dolerites obtain ; but in these cases the 

 ores seem to be the result of the decomposition of pyritous bands in 

 the rock, rather than a repetition of the conditions observed on the 

 Bowen river and near the Calliope Station. 



On Agate Creek, a tributary of the Gilbert river, a. local volcanic 

 outburst of no great extent is met with, the cavities in a thick band of 

 melaphyre, which forms a portion of this flow, being filled with agate 

 and carnelian .This resembles in general character the agate melaphyres 

 of the Yaal river in South Africa, the diamond-producing district. 



z2 



