574 CASTS OF RADIOLARIA IN PRE CAMBRIAN ROCKS, 



If Eozoon Canadense and allied forms be left out of considera- 

 tion, the above comprise, as far as we are aware, references to all the 

 more important papers relating to the microzoa of the Pre-Cam- 

 brian Rocks. 



3. Description of the Radiolaria. 



Obviously the two most important points to be proved in this 

 note are (a) that the supposed organisms are referable to radiolaria; 

 ami (b) that the rocks which contain them are of Pre-Cambrian 

 Age. 



If direct proof of the first is wanting, the question as to the 

 age of the rocks does not so much matter. We shall, therefore, 

 proceed first to quote evidence which, in our opinion, is strongly 

 in favour of the structures about to be described being referred 

 to the radiolaria, and afterwards we will deal with the question 

 of the geological horizon of the rocks which contain the radiolaria. 



Traces of the organisms referred by us provisionally to the 

 radiolaria occur at two localities, (a) Brighton, about 10 miles 

 S.S.W. from Adelaide; and (b) Cr3^stal Brook, about 140 miles 

 N. of the same city. At («) Brighton the forms provisionally 

 referred to the radiolaria occur scattered in great numbers 

 throughout a greenish siliceous limestone. This limestone in 

 places exhilnts well mai-ked oolitic structure. 



Thin sections of these rocks prepared by the students at the 

 Geological Laboratory, at the University of Sydney, show that 

 these supposed casts of radiolaria are partly chalcedonic and 

 opaque, partly replaced by lime and translucent The latter 

 types are invested in places with a black network, chiefly com- 

 posed of iron pyrites, the intimate structure of which is hai'd to 

 determine. Casts of what we consider to be the medullary shells 

 are most frequent, and are best preserved. A careful examination, 

 however, of the material surrounding these spherical translucent 

 bodies frequently reveals the presence of an outer nebulous ring, 

 sometimes showing a denticulated margin in cross section. (See 

 PI. .vxxix. figs. 5-6.) That these bodies are radiolarian casts and 



