42 ON FELSITES AND ASSOCIATED ROCKS. 



would have a practical effect on niiuing, as the discovery 

 of felsite would indicate the proximity of ore. The case 

 of the wliite inelai)hyre at Zeehaii, which favourably affects 

 the silver-lea<l lodes there, suggests the possibility of some- 

 thing similar ruling in the case of rhe Mount Read felsite. 

 But the two cases are not parallel. At Zeehan the lodes 

 in 4uesti(ni traverse the eruptive rock ; at Mount Read the 

 ore boelies are outside it. And it is difficult to see how the 

 latter W(nild affect ore deposition in the schists, when, as 

 appearances in<licate, the ore was deposited subsequently 

 to the foliation and metamorphism of both felsite and 

 schist. It is true that ore is found never very far away 

 from the felsite. A verj' natural way of accounting for 

 this is that schists and felsite are geographically associated, 

 and form together one mountain complex. 



A(/e of the Fehitc. 

 If our interpretation as set forth above l)e correct, the 

 geological age of the schists and felsite is the same for 

 both. There is no direct evidence of precise age yet avail- 

 able. The schists themselves in the vicinitj' of Mount 

 Read are argillites and phyllites, and occasionally retain 

 in places less altered remnants of slate, but no fossils 

 have been found in them. The most recent determinative 

 work done in this direction is R. Etheridge, jun.'s descrip- 

 tion of Mr. A. ]Montgomery's collection of Silurian fossils 

 from the limestones of Zeehan and the Heazlewood. Mr. 

 Etheridge says they " present both a lower and an upper 

 Silurian facies, but with a preponderating tendency towards 

 the latter." He thinks " it is not improbable that they 

 represent a series of beds homotaxially equivalent to the 

 lower portion of the upper Silurian.'' " Judging from the 

 succession, the Motmt Read and Mount Black schists are 

 somewhat older than the Zeehan series, and are probably 

 not younger than the lower Silurian. But great caution is 

 necessary here, as the evidence is of a negative character.. 

 The test of superposition is unreliable, as the persistent 

 easterly dip of the strata on the west slope of Mount Read 

 points to overfolding on a large scale, which has produced 

 an inverted succession of the beds. In any case, the felsite 

 is much older than any of our known granite rocks. 



Detcriuinatioii of iJie Felsite. 

 Anticipating for a moment the results of our micro- 

 scopical examination, we may say that the predominance 



* Description of a small collection of Tasmanian fossils. — R. Ethe- 

 ridge. jun. 



