BY T. W. EDGE WORTH DAVID. 565 



1000 feet in thickness. Thej^ ai-e capped by basalt. Xear their 

 junction with the limestone they are seen to be very much inter- 

 sected b}' eruptive dykes, porphyritic by augite. It may be 

 inferred from the circumstance that nearly all the dykes to the 

 east of the limestone are felsitic, while no felsite dykes occur to 

 the west of the limestone, that the basic character of the former 

 group of dykes is due to the eruptive rock having assimilated 

 much lime in its passage through the limestone bed, for as the 

 dip of the limestone is westerly at an angle of 60°, and the dykes 

 are nearl}^ vertical, they could not have reached the surface 

 without first passing through the limestone bed. The dark 

 shales are not distinctly cheity except where they are in close 

 proximity to the dykes. The cherty character of the beds in this 

 case is due therefore, I think, to contact metamorphism rather 

 than to silica derived from radiolarian 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 preserva- 

 tion in the cherty bands. Below the Jenolan Cave Limestone 

 are several hundred feet of dark indurated shales, greenish-gre}^ 

 argillites, reddish-purple shale and coarse volcanic agglomerates 

 with large lumps of Favosites, HeUolites, etc. The ax'gillites and 

 grey shales contain numerous casts of radiolaria, but in a very bad 

 state of preservation. 



3. Macroscopic and Microscopic Description of the 

 Radiolarian Rocks. 



The radiolarian rocks from Bingera and Barraba are hard red 

 jaspers, the base of which is very opaque even in thin section. 

 In places the red jaspers pass into a nearly white quartzite. 

 Such portions of the rock as approach quartzite and chalcedony 

 in character show scarcely any ti'ace of radiolaria, probably owing 

 to the shells having lieen completely disso]^"ed during the meta- 

 morphism of the rock. The opaque red jaspers, howe^'er, especiall}^ 

 those which have not undergone much metamorphism, contain 

 very abundant casts of radiolaria, so abundant as to make it 

 38 



