Vol. 55.] DEVONIAN ROCKS OF NEW SOUTH WALES. 39 



casts of radiolaria can be distinguished in it ; these are extremely 

 shadowy and indistinct, and can scarcely be differentiated from the 

 groundmass. I can see no evidence of metamorphism in the rock ; 

 it closely resembles some jaspery radiolarian cherts from the Culm 

 of Devon, and also some, of presumed Jurassic age, from California, 

 in which there is no question of contact-alteration. The samples 

 examined were : — 'No. 575,^ from Lower Bingara, and No. 573, 

 from Oakey Creek. 



In only one of the chert-specimens from the Jenolan Cave district 

 (jSTo. 569, McKeown's Creek) have I been able to recognize radio- 

 laria. This rock is a black chert traversed by a close network of 

 microscopic quartz-veins. The radiolaria are fairly abundant ; they 

 are in the condition of casts of microcrystalline silica, without 

 structure, and neither spines nor central tests are shown. The 

 rock is a radiolarian chert without indications of metamorphism. 



The other specimens (Kos. 568, 570) from the same locality as 

 the above are black, opaque, impure cherts, in which no organisms 

 could be seen, and the same may be said of a specimen from Cave 

 Creek (jSTo. 571) in which there is an admixture of fragments of 

 tuff. 



At Tamworth the chert associated with the dark siliceous lime- 

 stones is a hard, black, compact rock, very finely laminated, as shown 

 by lines of lighter and darker materials in the microscopic sections. 

 The matrix is filled with dark, minutely granular, or floceulent 

 material, considered as carbonaceous by Messrs. David & Pittman ; 

 but some of it may possibly be ferruginous. This dark substance is 

 either generally dispersed throughout the rock, or occurs in denser 

 patches and streaks. In thin section under the microscope the 

 rock is seen to be crowded with radiolaria, which appear as small, 

 clear spots, for the most part circular in outline, without definite 

 bounding- walls. Neither spines nor central tests are shown in the 

 great majority of the forms. The radiolarian casts are usually 

 infilled with cryptocrystalline silica, sometimes with fibrous chalce- 

 donic silica, and in exceptional instances with crystalline quartz. 



The radiolaria preserved in this dark chert are distinguished 

 from those in the intervening beds of dark siliceous limestone by the 

 absence of the latticed structure of the cortical test, and the nearly 

 complete disappearance of the radial spines and medullary test. It 

 can hardly be doubted that the chert forms were originally of a 

 similar character to those in the limestone interbedded with the chert, 

 and that their present differences arise from the different coDditions 

 of fossilization in the chert and limestone respectively. Judging 

 only from their present appearance, the simple, spineless forms in the 

 chert would be referred to groups quite distinct from those in the 

 dark limestone, with which, there is good reason to believe, they 

 were originally identical. 



In one specimen from Tamworth (No. 375, 4-5) a nodule of the 

 black chert is enclosed in a bed of tuff. The chert appears to be 



^ The numbers affixed to the specimens throughout this paper are those used 

 by Messrs. David & Pittman. 



