BY A. B. WALKOM. 99 



throughout a long period during which the Narrabeen and 

 Hawkesbury Sandstone Stages were deposited. The area of 

 deposition increased gradually during this time, the Hawkesbury 

 Sandstone Stage covering a much greater area than the Narra- 

 been Stage. During the early stages, volcanic activity appears 

 to have been in evidence, as indicated by the development of 

 tufFaceous rocks in the Karrabeen Stage. The thickness of the 

 sediments deposited in this basin reaches a maximum of about 

 3,000 feet, made up of a maximum of 2,000 feet of the Narra- 

 been Stage, and 1,000 feet of the Hawkesbury Sandstone Stage. 



The Hawkesbury Sandstone Stage is apparently conformable 

 with the Narrabeen Stage. The existence of breaks, which do 

 not show as very marked unconformities, must, however, always 

 be borne in mind. Writing on this point, Schuchert remarks,* 



" The easily seen, marked unconformities are of course 



accepted at full face-value; but the many more apparently con- 

 formable and yet broken contacts, the disconformities, are gener- 

 ally overlooked, or when seen are generally undervalued. ... In 

 regard to the breaks, the statement can be made that there are at 

 least ten disconformities for every known angular unconformity." 



These statements may be applied to both the Permian (Permo- 

 Carboniferous) and Mesozoic Systems of Australia, within which, 

 apparent conformity appears to be general, but where there are 

 quite a number of palseontological breaks. 



During the time in which the Narrabeen and Hawkesbury 

 Sandstone Stages were deposited, probably the rest of Australia, 

 which at that time was of greater extent than to-day, was dry 

 land. 



Between the Hawkesbury Sandstone Stage and the Wiana- 

 matta Stage there is a distinct break. The Wianamatta beds 

 have been deposited on the eroded surface of the Hawkesbury 

 Sandstone, which must, therefore, have been elevated, exposed 

 to denudation, and again submerged before the succeeding beds 

 were laid down. In addition, the extent of the Wianamatta 

 beds was much more limited than the Hawkesbury Sandstone. 



* Bulletin Geol. Soc. America, xxvii, 1916, p.497. 



