Yol. 55.] DEVOJTIAN ROCKS 0¥ NEW SOUTH WALES. 41 



to pieces when the radiolaria were being deposited at the sea- 

 bottom. 



In polarized light between crossed nicols, the etched sections for 

 the most part remain dark, but occasioually the faint outlines of 

 radiolaria in eryptocrystalline silica are visible, and also minute 

 polarizing particles of uncertain character. Microscopic cubes of 

 pyrites are not at all uncommon in the dark limestones, but I have 

 not observed any clastic inorganic fragments. 



The calcareous constituent of this radiolarian siliceous limestone 

 appears to be wholly in the form of crystalline calcite, and there are 

 no indications in the specimens examined of foraminifera or other 

 calcareous organisms which could have furnished the calcite. It is 

 possible that originally they may have been in the rock and subse- 

 quently obliterated, for in the coral-limestones which occur in part 

 of the Tamworth series (258 b) the coral-structure is now replaced 

 by calcite. However this may be, it seems clear that this siliceous 

 limestone must have been originally a radiolarian ooze, and that the 

 calcite which now infills the tests and interspaces of the organisms 

 was subsequently introduced, giving solidity and density to the 

 light, porous, siliceous aggregate. The rock when treated with acid 

 remains of the same volume after the lime has been removed, and 

 the porous residue, as we have seen, essentially consists of the 

 siliceous remains of radiolaria. 



From the etched sections of this rock about half the forms described 

 and figured in the following pages were obtained. 



(c) The Siliceous Claystones and Shales with Radiolaria. 



The only specimens of this description of radiolarian rock that I 

 have examined are from Tamworth (590, 625 25), where Messrs. 

 David & Pittman^ have ascertained that it is some thousands of 

 feet in thickness. The rock is greyish-brown, somewhat lighter- 

 tinted on the outer surface, compact, jointed, aud traversed in 

 places by narrow quartz-veins. It gives no reaction in acid. The 

 weathered surface of the rock is covered with minute grey specks, 

 barely visible to the naked eye ; these with a lens are seen to be 

 radiolaria, partially weathered out from the brownish matrix. 

 In some instances the lattice-structure, and more frequently the 

 radial spines of the organisms are preserved in position. A thin 

 section of the rock under the microscope shows a translucent ground- 

 mass of very minute, brownish, polarizing particles. The rock is 

 crowded with radiolaria, which, however, in the section show 

 neither structure nor spines ; they are now infilled with a brownish 

 material (whitish-grey by reflected light) of an uncertain character. 



As the authors of the foregoing paper remark, this claj'stone and 

 shale apparently contain as many radiolaria as the blnck chert, 

 while the chemical analysis shows a distinctly smaller proportion of 



1 These authors state that the claystones at Bingara aud Barraba are htho- 

 logically similar to those exposed at Tamworth. 



