5GG RADIOLAIIIA IN PALAEOZOIC ROCKS, 



evident that the racliolaria must in this case have contributed 

 very largely to form the rock. 



Under the microscope numerous spherical or oval bodies, from 

 •05 mm. to "215 mm. in diameter, are seen to be distributed through 

 the base. The outlines of the larger casts are jagged, the jDroject- 

 ing points representing casts in chalcedony of the openings in the 

 original latticed shell. Most of the smaller casts are probably 

 those of the medullary shell. The larger casts very frequently 

 occur in j)airs. Only in one instance was the original outer shell 

 of a radiolarian organism noticed. It was separated l)y an inner 

 ring of red jasper from the cast of the medullary shell. The 

 form appeared to be allied to Carposphcera. Some of the largest 

 of the casts, about "215 mm. in diameter, are probably referable 

 to Cenosphce7-a. Many of the radiolarian casts have participated 

 in the numerous minute faults to which the rock has been sub- 

 jected. The Tarn worth radiolarian rocks, as alread}' mentioned, 

 are partly thin siliceous limestones, partly argillites and black 

 cherts, partly massive coralline limestones. 



The black cherts do not appear to owe their silica entirely to 

 the radiolaria, but to have derived it largely from the thousands 

 of granitic sills with which they are so regularly intersected as to 

 give the appearance of interstratification. 



The casts of radiolaria in these cherty argillites are much 

 better preserved than those in the red jaspers, and also than 

 those in the black cherts of Jenolan. 



Many of them show distinct traces of the latticed structure of 

 the shell. The radiolaria, however, are in a far better state of 

 preservation in the thin siliceous limestones, which weather into 

 a kind of " rottenstone." On the weathered sui'face of this rock 

 the radiolaria can be very easily distinguished with a pocket lens. 

 Thin sections of the rock do not show much of the structure of 

 the shells under the microscope on account of the difference in the 

 respective refractive indices of quartz and calcite being insufficient 

 to show up plainly the structure of the radiolarian shells. The 

 best results were obtained by thinning slices of the rock to the 

 thickness of the full diameter of the larger radiolarian shells, and 



