GEOLOGY OF THE JENOLAN OAVES DISTRICT. 337 



The Triassic and Permo-Carborriferous strata do not occur 

 within the area dealt with here and will not be further 

 referred to; this applies also to the Mount Lambie Series 

 of Upper Devonian age. The sedimentary formations actu- 

 ally occurring in proximity to the caves are of (?)Ordovician 

 and Silurian age, and these are intruded by igneous rocks 

 of late Devonian age. 



(a) The (?) Ordovician Strata. 

 The Radiolarian Cherts. — These lie to the west of the 

 Cave limestone, and are well shown in the cuttings along 

 the Tarana Road in the immediate neighbourhood of the 

 Cave House. They vary from jet black to bluish-grey in 

 colour, are exceedingly compact and fine-grained, and rarely 

 exhibit stratification ; when the latter is visible it is marked 

 by a series of alternating thin layers of lighter and darker 

 material. 



Prof. T. W. E. David has already described these beds 

 as follows: — 



"Immediately overlying the limestone are fine-grained dark 

 clay-shales and argillites and black cherts. Mr. V. Wyburd, the 

 guide to the Caves, informs me that these must be at least 1000 

 feet in thickness. The dark shales are not distinctly cherty except 

 where they are in close proximity to the eruptive dykes. The 

 cherty character of the beds in this case is due therefore, I think, 

 to contact metamorphism rather than to silica derived from radio- 

 larian shells. Both the black cherts and the softer and less 

 siliceous dark grey shales abound in casts of radiolaria. The casts 

 are in the best state of preservation in the cherty bands." 



No attempt has been made on our part to determine the 

 actual thickness of these beds, but they are unquestionably 

 very thick, probably considerably more so than the 1000 

 feet mentioned above. We are also of opinion that their 

 cherty nature is independent of their contact with the 

 igneous dykes as they possess this character at distances 



V— December 1, 1915. 



