68 on some water-wopvn and pebble-worn stones. 



Discussion. 



Eev. Edwin Hill asked if the Author had any means of telling 

 how far chemical action had operated. 



Mr. HuLKE wished to know if there was an}- record of the positions 

 of the six stones given in the table. 



Prof. Hughes was desirous of learning whether the stones taken 

 into account were an average sample of the stones. 



Mr. Whitaker commented upon the rapid waste. 



Mr. Careuthers asked whether the rock was homogeneous. 



Eev. H. H. WiNWooD inquired as to the nature of the pebbles. 



The President wished to know whether from the percentages 

 taken some datum could be given for estimating the average loss 

 from the whole surface of the apron. 



The Author believed the action was principally abrasive, as there 

 was only a small proportion of lime in the stone which would be the 

 subject of chemical action. The weir was placed diagonally across 

 the river, and the stones referred to, which were average samples, 

 were tal-en from the apron at the upper end of the diagonal, where 

 the abrasive effect appeared to be greatest. The pebbles were prin- 

 cipally of quartzose description. The rock from which the stones 

 were taken was of a homogeneous character. In the case of stone 

 'No. 1, supposing the action to have been uniform, the abrasion would 

 represent a loss of nearly one foot three inches from the surface of 

 the stone as originally placed in the apron, and the others in pro- 

 portion. 



[In accordance with a request made by the Council, the Author 

 has added the following notes : — 



The stones referred to, which were average samples of the pebble- 

 worn stones which had been removed, were taken from the apron 

 of the weir within a distance of from 40 to 100 feet from the Island 

 end of the weir, where the abrasive effect appeared to be greatest. 

 The stones, however, forming the central portion of the apron, be- 

 tween 100 and 200 feet from the Island end of the weir, were 

 almost similarly abraded and perforated, and would also shortly 

 require to be removed. 



Taking the whole surface of the apron, the stones which were not 

 affected by the action of the pebbles were worn down by the action 

 of the water and ice passing over them for an average depth of 

 about 2 inches from their top faces. That might be taken as the 

 average effect of the water and ice alone passing over the weir in 

 the 43 years during which time the apron had been subjected to 

 the action of those forces. — January 20, 1891.] 



