president's AUDKESS SECTIO]^ C. ' 181 



west of Geelong have been described as gabbros and also as diabase. 

 According to Gregory enstatite-diabases, epidiorites, and amphibolites 

 occur. The latter consists of needles of green hornblende, zoisite, and 

 altered felspar. 



The Chemical Composition of the Heathcotian Rocks. 



Most of the diabases have suffered such chemical as well as 

 structural changes that chemical analyses would afford little evidence 

 of the original composition of tiie rocks. I know of only one analysis 

 of a rock, described as a greenstone (diabase) from Geelong, but 

 the exact locality is not stated. (3.) 



Greenstone (diahase), Geelong. 



SiOs 50-84 MgO lOOT 



AloOa ■12-92 K„0 1-83 



FeliOj 0-.52 ]S'a„0 tr. 



FeO ■ 6-99 H„0 ... 0-71 



CaO 14-35 



Total 99-13 



Silurian (Yeringian). — We have no evidence of volcanic activity 

 <Uu-ing any part of the Ordovician period above the Heathcotian in 

 Victoria. This is strange, as andesites have been referred to the 

 Ordovician near Bathurst, in N.S.W., and in many parts of the world 

 vrdcanicity was vigorous throughout the period. This period of 

 quiescence in Victoria appears to have continued throughout the 

 lower or Melbournian horizon of the Silurian. In the upper or 

 Yeringian series, however, Chapman (38) has described the microscopic 

 characters of some limestones from the Thomson T?iver district, in 

 Gippsland, in the matrix of one of which under the microscope he 

 recognises flakes of biotite and pale green chlorite, and here and there 

 some contoi'ted bands of tuffaceous fragments. He says " this clearly 

 suggests the outburst of tuffaceous andesitic ejectamenta contem- 

 ])oraneously with the deposition of the limestone, as illustrated, for 

 example, by the calcareous organic tuffs forming in the proximity of 

 volcanic islands at the present day." This is so far the only evidence 

 we have of vulcanicity in the Silurian rocks of Victoria, and even in 

 this case the fragments may possibly be detrital in origin. 



DEVONIAN. 

 Lower Devokiax (1) 

 " sxowT river porphyries." {See Plate 4.) 

 Geographical Distribution and Physiography. 

 This group of Volcanic Rocks was first described and its geo- 

 graphical extent first indicated by the late Dr. A. W. Howitt (43). 

 Later information by Murray (46), 0. A. L. Whitelaw (78), and 

 Ferguson (75) has added something to our general knowledge of this 

 formation, but in ali essentials no advance upon Howitt's reports of 

 1876 and 1877 has been made. In view of the interest and import- 

 ance of this series it is curious that Howitt never published any later 

 work on this formation. Apart fi-om his early general description the 

 petrographical characters of the rocks remain undescribed, and as far 

 as I know no chemical analyses have been made. 



The geographical development of this series lies in the moun- 

 tainous district of N.E. Victoria, and the largest area is a belt of 



