BY T. W. EDGBWOKTH DAVID. 569 



The associated fossils prove the radiohxrian rocks, at Tamworth 

 at all events, to be homotaxial with the Burdekin Formation of 

 Queensland. Mr. R. L. Jack, the Government Geologist of 

 Queensland, and Mr. R. Etheridge, Junr., consider the age of the 

 Burdekin beds to be Middle Devonian. 



5. Deductions. 



(i.) In New South Wales there is a great development of rocks, 

 chiefly argillites, cherts and jaspers, foi'merly considered to be 

 unfossiliferous, but now pi'oved to be formed largely of the shells 

 of marine organisms, the radiolaria. 



(ii.) The geological horizon of these rocks is probably Middle 

 or Lower Devonian, perhaps Siluro-Devonian. 



(iii.) The cherty character of some of the rocks containing the 

 radiolarian casts is due rather to the introduction of silica 

 secondarily from eruptive dykes and sills than to the silica con- 

 tained in the radiolarian shells. 



(iv.) The preservation of the radiolarian casts in the black 

 cherts is chiefly due to the silicification and indui-ation super- 

 induced by contact metamoi'phism. 



(v.) This contact metamorphism took place some time between 

 the close of the Carboniferous Period and the commencement of 

 the Permo-Carboniferous Period, and was the I'esult of the 

 intrusion of sills and d3'kes of granite. 



(vi.) (a) The presence of thick beds of coralline limestone inter- 

 stratified with the radiolarian rocks, and {b) the vast thickness of 

 the radiolarian beds (several thousand feet being formed within a 

 single epoch of one period of geological time) render it improbable 

 that the rocks were formed in \'ery deep seas. This agrees with 

 Professor SoUas' recent observations on the 'Soapstone' of Piji, con- 

 .sidered by Brady to be of deep sea origin, but now proved to have 

 been deposited in shallow water. At the same time the absence 

 of conglomerates (with the exception of the volcanic agglomerate 

 at Jenolan) from the radiolarian beds and the abundance of inter- 

 stratified limestone indicates deposition in tranquil water at some 

 distance from the shore. 



