PHYSIOGRAPHY OF THE AUSTRALIAN ALPS. 379 



Lower Silurian can be maintained, recent examinations by the 

 author on the Gibbo Kan;iifes in the former, and tlie ranges south 

 of j\[andiliging in the latter, yielding obscure stems and leaves of 

 a fuciod of probal)ly Upper Silurian facies. 



The British equivalents of the Lower Silurian sediments of the 

 Australian Alps are the Llandeilo Flags and Bala rocks in Wales, 

 accoixling to the palaeontological determinations of Prof. McCoy. 



As regards the Upper Silurian, the great folding and alteration 

 to which the whole of the Lower and Upper Silurian sediments 

 have been subjected, renders the determination of a true strati- 

 graphical liorizon a work of difficulty. There are certain limestone 

 bands, conglomerates, and shales, presenting such well-marked 

 lithological characters, and also containing such distinctive fossils 

 that their position in the series can readily be determined. There 

 are two localities where such "distinctive rock-masses occur — one 

 in the Mitta Mitta valley at Gibbo River, and the other in the 

 Hume valley at Limestone Creek. 



In the Mitta Mitta valley, between the Gibbo River and the 

 Wombat Creek, there are extensive beds of conglomerate, shale 

 and bands of hard blue crystalline limestone — the Gibbo River 

 beds — which contain fossils of an Upper Silurian facies. 



The limestone bands form lenticular masses representing holloAvs 

 in the ancient sea floor. The adjoining shales are very much 

 corrugated in places, and where they adjoin certain intrusive poi'- 

 phyrics are converted into hornfels. 



The accompanying Section A.-B. (PI. XXX.) across a limestone 

 'cliff' fi'om the Morass Creek, explains the sequence of the rock- 

 masses and their relations to the overlying Tertiaiy basalts in situ. 



On the Limestone Creek (Pi. XXTX.), at the head of the Murray 

 River, are bands of blue limestone and white marble, forming the 

 Limestone Creek beds, whose stratigraphical relations to certain 

 adjoining porphyries of a fragmental character have been well 

 worked out, and the age of the latter determined, by the admirable 

 and interesting petrographical researches of Mr. A. W. Howitt, 

 as Devonian. From the position of the limestone beds, &c., and 

 their relation to the porphyries their probable age was suggested as 

 Upper Silurian. It fell to the lot of the author to contirm these 

 views by tlie discovery of undoubted Upper Silurian fossils at 

 Stony Creek, a small tributary of liimestone Creek. During a 

 preliminary examination of these marble beds some years since 

 the sectional notes were obtained which may be found in my 

 paper, " Notes on a Geological sketch-section through the Aus- 

 tralian Alps," read before the Royal Society of South Australia 

 on February 5, 1884. 



The Limestone Creek caves ax-e, although of no great extent, 

 yet sufficiently large to be of interest. I have already described 

 the salient features of these caverns elsewhere,* so that I need 



* On Caves perforating Marble Deposits, Limestone Creek. Trans. K. Soc. Victoria, I8S3. 



