340 Ernest W. Slrafs .- 



sistinfT chemically of silicate of alumina, water and oxides of 

 chromium and aluminium. 



10. — T'he Age of the Rocks of Hbathcote. 



(a) Fossils of the Dinesus Beds. — The evidence for the age of 

 these rocks rests on a few fragments of trilobite, some badly 

 preserved braicKiopod fragments, and some obscure markings, 

 thought at first to be graptolites of the Bryoj^rapins type, but 

 later regarded as probably algal in chairacter. These latter and 

 the brachiopods afford little help in determining the age of the 

 rooks. The trilobites, according to Mr. Etheridge, junr. (op. cit.), 

 have Cambrian affinities. Professor Gregory, with mnre material 

 to work on, regards the fragments as contaiining two genera, 

 both new, and on the whole probably of Lower Ordnvician age. 

 In rock sections of some of the Dinesus beds I have noticed 

 some minute circular chalcedonio arcias, some apparently tubular, 

 Avhich may be cross sections of sponge spicules, but no definite 

 structure cam be made out. Other areas suggest the possibility 

 that Radiolaria may be present. 



From a locality near the junction of Dinesus beds and the 

 normal Ordovician I found on splitting open some shales several 

 casts of large spicules intersecting at right angles, characteristic 

 of Pro/osp(>>ii:^ia a form which in Britain I believe is only found 

 in Cambrian rocks. In view of the occurrence of Bryograptus in 

 Victoria associated with Lower Ordovician forms, not much 

 weight can, I think, attach to the occurrence of these sponge 

 spicules in the Dinesus beds. Their presence probably would not 

 necessitate placing the rocks with the Cambrian seanes' if the 

 evidence of the trilobites points to the Lower Ordovician age of 

 the rocks. 



(6) Fossils of the Ordovician Rorl-s. — I have above staited ray 

 reasons for dissenting from the separation of the beds containing 

 the Dinesus fauna from the beds which succeed them, and which 

 have been, on lithological grovmds, regarded as Ordovician. 

 Sections were made from a rock (No'. 634) (PI. XV]., Fig. 4) occurring 

 in the Ordovicians .0 feet west of the outcrop of tlie breceiated 

 coiiglonierate on tlio roiul running W. from the Murray road 

 towards the diabase (if Tranter's paddock. Under the micro- 



