BY T. AV. EDGEWORTH DAVID. 567 



then etching the slice with dilute hydrochloric acid. Much of the 

 structure can be developed in this way as shown on Plate xxxvii., 

 from a microphotograph kindly taken for me b}'^ Mr. W. F. 

 Smeeth, M.A., B.E., Assoc. R.S.M. 



As I ha\e forwarded some of this matexial to Dr. Hinde, who 

 has kindly undertaken to describe the radiolaria specially, I will 

 not attempt to do more than mention that some of the commonest 

 forms in the Tamworth rock are figured on Plate xxxviii. 



It is obvious that the legion of the Spumellaria is much better 

 represented than that of the Nassellaria. Fig. 7, Plate xxxviii. 

 appears to represent a Xvphcajjluura, but the spines ap];)ear to be 

 perforated by openings, giving the shell somewhat the appearance 

 of PiprMetella (Challenger Reports, Radiolaria, Vol. xviii. PI. .39, 

 Fig. 6). Fig. 2 shows the inner and outer shells fairly well pre- 

 served, and is probably a Haliomma. Fig. 5 perhaps represents a 

 TJieodiscus; and Fig. 9 perhaps a Staurolonche or an Astromma. 



As regards the state of preservation of the shells the original 

 siliceous skeleton is for the most part represented, but is some- 

 times replaced by iron pyrites. Often internal casts alone, in 

 chalcedony, are all that remain to tell of the former presence of 

 the radiolaria. Spicules of hexactinellid sponges are visible in 

 places, in this rock. The radiolaria are so abundant as to give 

 this rock, when etched, the appearance of a Barbadoes earth. It 

 was probably in its original condition a radiolarian ooze. 



At the Jenolan Caves, as already stated, the ^radiolarian casts 

 are best preserved in the black cherts, where they are very 

 numerous. Numerous traces of radiolaria can also be detected in 

 the soft argillites and hardened clay shales. 



The radiolarian casts are in a better state of preservation in the 

 black cherts than in the red jaspers of Barraba and Bingera. 

 Latticed structure is, however, scarcely anywhere to be seen. 

 Such slight traces of it as do occur are preserved in the form of 

 opaque black fragments of network entangled in a sub-translucent 

 cryptocrystalline base, as seen in thin sections under the micro- 

 scope. 



