Fetroloyy of the SUurian Sediments. 47 



luuscovite, and carbonaceous material in a clayey groundniass. It 

 is very even grained. 



(e). South Yarra Specimen. — No. was obtained from the cliff 

 section on the left bank of the River Yarra just below the Church 

 Street Bridge. It is a hard, tougli, cherty looking rock. Under 

 the microscope it shows small angular fragments of quartz, and a 

 little muscovite, tourmaline, and rutile, in a very tine sericitic^ 

 groundmass. It seems to have suffered considerable pressure. 

 Secondary limonite is also present throughout the rock. 



4. — Metamorphism. 



Practically no metamorphism of the rocks has occurred. Al- 

 though intensely folded and fractured no cleavage is developed. No- 

 contact metamorphism has taken place. 



The series is intruded by two series of dykes, one basaltic and 

 tlie other of the nature of quartz por2:»hyry. The only alteration 

 is that due to the percolation of solutions containing iron derived 

 from the dykes. The sediments are largely impregnated with 

 limonite at the contacts. 



5. — Deposition of the Sediments. 



C^hapmani refers the Melbournian sediments to a warm shallow 

 sea on the evidence of the prevalence of the brachiopod Lingula, 

 and the almost complete absence of the corals. This view is sup- 

 ported by the writer. The general fineness in grain suggests that 

 the sediments were deposited some distance from the shore. Con- 

 glomerates occurring at Keilor, about 10 miles from Melbourne, 

 probably i-epresent the nearest part of the shore line sediments of 

 the Silurian sea. 



The admixture of relatively coarse sand with the fine materials, 

 of the shales suggests that the sediments may have been laid down 

 under flood conditions or under rapid variations in the strength of 

 the currents. This view is supported by the relative thinness of the- 

 beds and the rapid alternation of sandstone and shale. 



6.— Origin of the Constituent Minerals. 



Two sources are possible for the material of the sediments : — 



i. They may have been derived from a pre-Silurian igneous rock ; 



or, ii., They may have been derived from a pre-Silurian sediment. 



Both sources are also possible. 



1. "On the Palaeontology of the Silurian of Victoria." Proceedings of Section C, p. 213, 

 Australasian Association for the Advancement of Science, Melbourne, 1913. 



