152 Robert A. Kehle: 



became the dominant factors in the erosion of the systems, and are 

 essentially ranges formed by erosion. When the lava in the Gem- 

 brook bottle-neck disappears the drainage of the Middle Yarra may 

 again find an outlet in this direction. - The watersheds between the 

 lateral valleys are still the most conspicuous ranges and spurs of 

 the systems, and trend north and south; even in the Yarra basin 

 the trend of the ridges between the laterals on its south side dis- 

 close to some extent a southerly gradient. ,. 



Isolation of residuals. 



One has only to travel in the direction of the infilled valleys to 

 realise that residuals are separated by the valleys of the cross 

 streams, and that when allowance is made for the amount of lava 

 removed by them, an approximate idea of the length and extent of 

 the confined lava fields may be formed. For instance, if we taKe 

 the confined lava field that occupied the valley of the Pre-Older 

 Basalt stream that originated somewhere near Mt. BuUer and 

 trended towards Flinders, we encounter a number of residuals 

 isolated by east or west valleys — the valleys of the cross streams. 

 In the trend of this ancient valley, the Mt. Buller residual is sepa- 

 rated from Klingsporn's residual by the Howqua. Klingsporn's 

 residual is separated from the Woods Point residual and "uncov- 

 ered residual " by the valleys of the Jamieson and Goulburn, the 

 AVoods Point residual and ''uncovered residual" is separated 

 from the Mt. Leckie residual by the valley of the Yarra, the Mt. 

 Leckie residual is separated from the Neerim residual by the valley 

 of the Latrobe, and the Neerim residual is separated from the 

 Drouin residual by the valley of the Tarago. Southwards from 

 Drouin the lava may exist as a connected sheet; at any rate, it be- 

 longed to an extensive lava field with much of the erosion hidden 

 by block faulting and masked by recent deposits. Other examples 

 could be cited, but the geological map of a district will disclose 

 more at a glance than a detailed description. 



When the point of time at which any of these cross streams 

 actually breached the confined lava plain is determined accurately, 

 a connected geological history of not only the evolution of the 

 -stream systems, but also the accumulation of deposits in tlieir 

 valleys will be possible. , 



The repeated rejuvenation of the streams that have accomplished 

 the breach has in many cases been the result of block faulting, but 

 ^caution is necessarv lest one should ascribe to a fault what may be 



