T. W. E. David— "Varve Shales" of Australia. 115 



Art. VI. — The "Varve Shales" of Australia*; by 

 T. AY. Edgewoeth David, Professor of Geology and 

 Physical Geography, University of Sydney. 



1. Varve Shales of Lower Carboniferous Age. 



Varve shales inter stratified with beds containing the 

 Carboniferous fossil ferns Cardiopteris and Rhacopteris 

 have recently been described by Mr. Sussmilch and the 

 author. 1 They occur chiefly at two horizons separated by 

 100 feet to over 600 feet of strata. 



The specimen here exhibited is a laminated shale — 

 laminae of deep brick-red alternating with thinner laminae 

 of pink to light grey tint. It exhibits beautiful examples 

 of minute contemporaneous contortions, affecting inter- 

 mediate laminae, but leaving those above and below the 

 contorted zone quite undisturbed. The shales are inter- 

 stratified with tillites, and glacial conglomerates and 

 glaciated pebbles occur in turn. It is considered, there- 

 fore, that they are genuine Paleozoic "varves" as 

 defined bv Professor Gerard de Geer, 2 and described by 

 Robert \Y. Sayles. 3 



The paired laminae are very conspicuous and on the 

 assumption that each pair of laminae has an average thick- 

 ness of two-tenths of an inch and that each pair repre- 

 sents one year's deposition of sediment, the authors have 

 roughly counted the number of pairs in such sections as 

 are available and estimated that the aggregate of 220 feet 

 of varve shale required a period for deposition of at least 

 4,000 years. This is a minimum estimate. For example, 

 in the specimen exhibited, which is 84 millimeters thick, 

 there are twelve pairs of laminae. If this were the aver- 

 age thickness of the paired laminae the time needed for 

 their formation would be nearer 10,000 years. 



The authors ascribe the contemporaneous contortions 

 in these "varves" to the movement of ice in some form 

 tending to develop horizontal gliding planes. 



* Eead at the First Pan-Pacific Scientific Conference at Honolulu, August, 

 1920. 



1 Sussmilch, C. A. and David, T. W. Edgeworth, Carboniferous and Permo- 

 Carboniferous rocks, New South Wales, Journ. Koy. Soc, New South Wales, 

 vol. 53. pp. 270-273, 1919. 



2 de Geer, Gerard, A geochronologv of the last 12,000 years, Compt. Kend. 

 Cong. Geol. Intern. Sess. II, pp. 211-253, 2 plates, 1910, 1912. 



3 Sayles, Eobert W., The Squantum tillite member of the Eoxbury con- 

 glomerate series. Memoirs, Mus. Comp. Zool. Harvard, vol. 47, No 1, 1919. 



