54 W. R. BROWNE. 



the geological map of the State reveals the existence of 

 large elongated masses of acid igneous rocks in among the 

 sedimentary formations, which themselves have been 

 folded along sub-meridional axes. The author lias observed 

 this sort of thing in the neighbourhood of Cooma, where 

 the Silurian and Ordivician sediments, chiefly shales and 

 slates, limestones and phyllites, upturned, steeply dipping 

 and often much cleaved, have been invaded by long sill-like 

 masses of gneissic granite and quartz-porphyry. The 

 courses of the streams, the Murrumbidgee and the smaller 

 rivers, are markedly influenced by the existence of these 

 hard bars, which have themselves not improbably been 

 brought into being as a result of the folding movements 

 which affected the sedimentary strata. It seems to the 

 author that this fact of lithological character and dispo- 

 sition of the geological units has not been given sufficient 

 weight by those who have studied the physiography of this 

 region, and it does not seem impossible that part, at least, 

 of the valley of the Upper Murrumbidgee, which has been 

 explained by Sussmilch and Taylor as due to sub-meridional 

 trough-faulting, may really be the result of the erosion of 

 a terrane composed of units of widely varying hardness. 



Among the most important factors which may modify a 

 river's history are to be placed rock-cleavage and jointing, 

 and the influence of these is clearly to be seen in some 

 parts of the area already referred to. The country is in 

 part composed of schists and micaceous gneiss with con- 

 spicuous jointing almost at right angles to the nearly 

 vertical schistosity, and the courses of many of the tribu- 

 taries of the Murrumbidgee, and to a less degree, that of 

 the main stream itself, are determined by these two 

 structural features, being very tortuous and formed of a 

 succession of right-angled bends, the factors determining, 

 the channels being now the schistosity, now the jointing* 



