Dr. G. J. Hinde — South Australian Chert. 115 



question is termed/ has hitherto been met with only in the Upper 

 Cretaceous of the United States ; and there is so much agreement 

 between the teeth of the typical B. Harlani (Meyer) ^ of the New 

 Jersey Greensand and the tooth now described, that the latter may 

 be provisionally recorded as Bottosauriis helgiciis. So far as the 

 writer's observations have extended, this is chai'acterized by the 

 presence of a small eminence at each end of the dental crown, 

 and the sharply-rounded — not keeled — form of its median longi- 

 tudinal line. 



IV. — Note on Specimens of Cherty Siliceous Eock from South 



Australia. 

 By George Jennings Hinde, Ph.D., F.G. S. 



SOME specimens of Cherty rock forwarded to England for deter- 

 mination some years since by Mr. H. Y. L. Brown, F.G.S., 

 Government Geologist of South Australia, have lately been submitted 

 to me for examination by my friend Dr. H. Woodward, F.R.S., and 

 their structure, as shown in thin sections, seems to me sufficiently 

 interesting to be recorded. The specimens in question are of a 

 light- or yellowish-grey tint, with a waxy lustre, in some portions 

 compact and in others minutely porous, which makes them light to 

 the feel. The rock is hard, scratching glass readily, and there is no 

 reaction with acid. Numerous negative casts of fossils are present 

 in it, the most abundant are those of a foliate Polyzoon probably of 

 the genus Betepora, but casts of small turreted Gasteropods and 

 bivalve mollusca are also not uncommon. In all cases the original 

 calcareous tests of these organisms have been dissolved and removed, 

 leaving impressions of their form in the translucent siliceous matrix ; 

 in some instances the casts have subsequently been filled up by 

 silica. 



Thin microscopic sections of the rock show that minute angular 

 quartz-grains are scattered throughout the cherty matrix, but the 

 principal feature is the occurrence of detached sponge-spicules which 

 in places are heterogeneously crowded together in the rock. These 

 spicules are by no means well preserved ; they are mostly frag- 

 mentary and in various stages towards dissolution. Where the 

 process has extended furthest, the substance of the spicule has 

 disappeared, and there is either an empty cast, or merely the axial 

 canal of the spicule now infilled with a dark substance. In all 

 parts of the rock-section the former presence of spicules can be 

 detected, although it is only in certain places that the spicules them- 

 selves remain in their originally crowded condition. The matrix in 

 which the spicules and quartz-grains are imbedded appears to be 

 mainly of amorphous or opal silica, nearly entirely neutral to 

 polarized light between crossed Nicols, and it is principally in the 

 form of very minute globules or discs usually aggregated together 



1 L. Agassiz, Proc. Acad. Nat. Sci. PMlad. 1849, p. 169. 



2 J. Leidy, Cretaceous Kept. United States (1865), p. 12, pi. iv. figs. 19-23 ; 

 pi. xviii. figs. 11-14. For further references to Bottosauriis, see E. D. Cope, Yert. 

 Cret. Form. West (Eep. U.S. Geol. Surv. Territ. vol. ii. 1875), p. 253. 



