BY A. J. SHEARSBY. 283 



These beds of contemporaneous lavas and tuffs which cover 

 such a large area of the Yass district, and are intercalated between 

 the Upper Silurian and Middle Devonian strata, are of such a 

 nature that I feel very much disposed to look upon them as 

 forming the only record we have in the Yass district of the Lower 

 Devonian period. The close of the Upper Silurian period seems 

 to have been attended by vast volcanic disturbances, which kept 

 up in a state of great activity till the Middle Devonian was 

 ushered in. 



The thick beds of shale at the base of Clear Hill are also 

 probably tufaceous in nature, and the layer of felsitic tuff, which 

 divides the shale into two parts, is also indicative of intermittent 

 volcanic disturbances during the laying down of the Middle 

 Devonian sediments. 



This question of the age of the volcanic rocks of Yass being 

 Lower Devonian is one, I think, that could be settled if a careful 

 geological survey were made of the country comprised in the 

 parishes of Cavan, Waroo and Boambolo. 



I was unable to make a collection from the southern portion 

 of the parish of Boambolo, but a few specimens, stated to be 

 from that locality, were handed to me for inspection, and I 

 recognised them as being similar to those of the Glenbower 

 formation, showing that the outcrop of Upper Silurian rocks was 

 continued on the other side of the river. 



I find that if the proposed weir be built at Barren Jack, lower 

 down the Murrumbidgee, the water will be dammed back some 

 distance above the Glenbower anticline, so that portion of the 

 outcropping Silurian rocks will be submerged; and it is advisable, 

 in the event of that water conservation scheme being carried out, 

 that any examination of the geological features of the Cavan 

 district be entered into at an early date, otherwise an interesting 

 section of the country will be hidden from view. 



The sketch map illustrating this short paper on the geology of 

 Cavan must be treated as a sketch only, for it was impossible to 

 make it complete, considering the fact that, having no assistance 

 during the short flying visits I was able to make to the locality. 



