120 PROCEEDINGS OF SECTION C. 



4._MOTE ON THE GEOLOGY OF JENOLAN. 

 By C. A. SUSSMILCH, F.G.S. 



Abstract. 



The area referred to is a tableland (part of the Central Tableland o. 

 New South Wales), the surface of which has an elevation of about 

 3,800 feet, and i- a peneplain cut out of lower Palaeozoic rocks. 

 Residuals of an older peneplain rise above this level and culminate 

 in Mt. Bindo, which has an elevation of 4,460 feet. This tableland; 

 in common with all the tablelands of New South Wales, was pro- 

 bably elevated to its present altitude at the close of the PKocene 

 Period ; since then it has been considerably dissected by the 

 development of numerous valleys and gorges during the present 

 cycle of erosion, which are still in the youthful stage of their 

 development. 



The oldest rocks in tne region are of Silurian age, and consist 

 of limestones, slates, radiolarian cherts and claystones, and rhyolite 

 lavas and tuffs, all of which are strongly folded and tilted. One 

 limestone bed has a thickness of upwards of 400 feet, and in this 

 occurs the Jenolan Caves. The Umestone is composed of the 

 remains of various marine organisms, including Corals, Crinoids, 

 Brachiopods and Mollusca. The rhyolite flow has a thickness of 

 about 300 feet and evidences the occurrence of extensive submarine 

 volcanic eruptions during the Silurian Period. The occurrence of 

 fossiliferous Devonian quartzites and claystones was also referred 

 to. It was then shown that the Silurian and Devonian strata has 

 been subjected to extensive orogenic earth-movements at the close 

 of the latter perod and strongly folded ; this was accompanied by 

 large intrusions of granite and quartz-porphyry. With regard to 

 these intrusions, the author expressed the opinion that the field 

 evidence favoured Daly's theory of " Overland Stopeing." 



The denudation of this region and the production of an 

 extensive peneplain during the Carboniferous and Lower Permo- 

 Carboniferous Period was then dealt with, followed by a description 

 of the subsequent deposition of the Upper Permo-Carboniferous and 

 Triassic strata. The paper concluded with a reference to the out- 

 pouring of basaltic lavas during the Tertiary Period. The paper 

 was illustrated with a number of specimens and diagrams. 



5.— NOTE ON THE LIMITATIONS OF DE CHAULNE3 METHOD OF 

 DETERMINING REFRACTIVE INDEX. 



By LEO. A. COTTON, B.A., B.Sc, Linnean Maclaiy Fellow in Geology. 



This note was prepared about three years ago at the suggestion 

 of Dr. W. G. Woolnough, whom I wish to thank here for his 

 encouragement in its preparation. 



