.A. J. Shearsby — On Operculate Corals. 547 



The foregoing replacement hypothesis would appear to supply 

 a fairly reasonable explanation for all the eccentricities of laterite. 

 When it was first developed by the writer, it was thought to be 

 entirely novel, but investigation of the literature of the subject has 

 shown that several writers have suggested the alternation of wet 

 and dry seasons as a predisposing cause ^ ; while Hislop ^ says that 

 '' the ferruginous matter coming up from among the metalliferous 

 strata, might, by the agency of water, have impregnated every 

 decaying rock on the surface, which, with the subsequent infiltration 

 of rain, would then present the appearance of laterite, as we now 

 find it"! 



IV. — Notes on the Operculate Madreporaria ,Rugosa from 

 Yass, New South Wales. 



By A. J. Sheaksby. 



(PLATE XXVI.) 



rpHE town of Yass, N.S.W., is situated on the Southern Eailway 

 JL about 190 miles from Sydney. It is the centre of one of the 

 most picturesque districts to be met with on the journey from 

 Sydney to Melbourne. The surrounding country is a veritable 

 geological and palseontological paradise for the collector, who may 

 depend upon turning up some new and interesting specimens every 

 trip he takes to the district. The town itself is built on the Upper 

 Silurian rocks, which about two miles distant, at a bend in the Yass 

 River known as Hatton's Corner, offer to the palaeontologist one of 

 the most prolific collecting -grounds in Australia. Amongst the 

 first to investigate these rocks were the Rev. W. B. Clarke^ and 

 Mr. C. Jenkins,* whose work was followed by a survey in 1882 by 

 the Mines Department.® Further good work was done by Mr. John 

 Mitchell ^ whilst stationed in the district ; but even at this late hour 

 there is a large amount of interesting work awaiting completion. 



About ten miles to the south and south-west of the town are found 

 the vast accumulations of Middle Devonian deposits, which are 

 separated from the Upper Silurian rocks by a thick bed of lavas and 

 tuft's which I believe to be the result of volcanic action during the 

 Lower Devonian period.' 



1 Broughton, Joui'n. As. Soc. Bombay, vol. v (1857), p. 639; Lapparent, Traite 

 de Geologie, p. 1611. 



2 Journ. As. Soc. Bombay, vol. v (1857), p. 63. 



* W. B. Clarke: " Kemarks on the iSedimentary Formations of N.S.W.," 

 4tli edition, 1878. 



^ C. Jenkins, "Geology of Yass Plains": Proc. Linn. Soc. jS'.S.'W., vol. iii, 

 pt. 1 (1878), p. 26. 



* T. W. E. David, " Report on the Fossiliferous Beds, Yass " : Ann. Eep. Dept. 

 Mines N.S.W., 1882 (1883), p. 148. 



6 J.Mitchell, " Notes on the Geology of Bowning " : Proc. Liun. Soc. X.S.TY. (2), 

 vol. i, pt. 4 (1887), p. 1193. " The Geological Sequence of the Bo\\'uing Beds, 

 N.S.W. " : Report Austral. Assoc. Adv. Science, vol. i, 1888 (1889), p. 291. 



" A. J. Shearsby, " On tlie Occurrence of a Bed of FossiJiferous Tuff, etc. " : 

 Proc. Linn. Soc. N.S.W., 1905, p. 275. 



