[Proo. Rot. Soc. Victoria, 29 (N.S.), Part I., 1916]. 



Art. 111. — The Petrology of the Silurian Sediments near 

 Melbourne. 



By W. G. LANGFORD, B.Sc. 



Kernot Reseax-ch Scholar in Geology, Melbourne University. 



(Oommnnicated by Professor E. W. Skeats, D.Sc.) 



1.— Introduction. 



Silurian sediments form the base rocks of the city of Melbourne. 

 Typical outcrops are exposed along the River Yarra from Princes'' 

 Bridge upwards, in Hawthorn and Kew, and also in the northern 

 and western suburbs. Coburg and Moonee Ponds. Carefully 

 selected specimens were taken from most of the outcrops. The rocks 

 are remarkably uniform in megascopic and microscopic characters 

 over the whole of Melbourne, so that the comparatively small col- 

 lection of specimens may be taken as representative. 



The rocks consist entirely of shales, mudstones and sandstones,, 

 thinly bedded. The beds average one or two feet in thickness. The 

 shales and mudstones are loosely compacted but in certain cases are 

 firm and tough where silicified. The sandstones contain much 

 aluminous material. In some cases they are partially cemented 

 into semi-quartzites by secondary silicification. In most cases the 

 cement consists of fine clayey material, in which case the rock 

 weathers down to a soft, porous, crumbly sandstone. The quartzite 

 beds stand out in prominent relief from these. Tliere has been an 

 extensive replacement of the sandstones by limonite along the 

 bedding planes. 



These sediment.s are steeply folded and fractuied around Mel- 

 bourne. In general the strike is northerly, but owing to the- 

 complications of movements, such as sagging and pitching of the 

 folds, and to the presence of numerous fractures, the strikes vary 

 greatly from that of the main fold axes. 



Some fairly large faults occur. In the bed of the Merri Creek 

 at Coburg (see " Description of Sections," Rock No. I.) there occurs 

 a well-developed fault breccia composed of angular fragments of 

 .sandstone, resembling the neighbouring Silurian sandstones, set in 

 a matrix of the same material, and now almost entirely replaced 

 by limonite. Small displacements are very widespread. 



