1906-1907.] 505 



nature of the rocks in the district is important. For example, 

 if the rocks in the gathering area be impervious clays or slates, 

 rain as it falls is carried off seaward immediately, with the 

 result that in the rainy season the brooks and rivers have a 

 very strong flood-flow, but are low in the dry season. A 

 satisfactory and constant water supply can only be got from 

 such a district by excavating large reservoirs, in which the 

 surplus water of the rainy season may be stored for summer use. 

 When the rocks in the drainage area are of a more porous type, 

 much of the rainfall percolates into the crust, issuing again as 

 springs, whose waters swell the volum.e of the streams and 

 rivers in the dry period. In such a district the flow of surface 

 water is more constant, and as a result there is much less need 

 for large reservoirs than in the case of an impervious area. In 

 estimating the yield of water from any drainage area we must 

 deduct from the total rainfall the loss from evaporation and 

 from the demands of vegetation during the growing period. 

 In all cases where surface waters are liable to contamination 

 from agricultural and other operations, such waters must undergo 

 a thorough filtering process. The lecturer then dealt with the 

 class of rocks, such as sandstone and limestone, favouring the 

 accumulation and suppHes of underground waters, and the type 

 of geological structure suitable for the connection of the various 

 kinds of springs, and the conditions under which boring for 

 water by artesian wells proves successful, illustrated by refer- 

 ence to a typical section through the County of Essex. Mr. 

 Duncan then treated on the disadvantages of the use of under- 

 ground water, as it is hard, and contains much mineral matter 

 in solution. Wells, sewage, &c., in rural districts, and how 

 contamination may be prevented, &c., were described. In con- 

 clusion, the lecturer referred to the subject of water-divining, 

 with especial reference to the Holywood case, and gave it as his 

 opinion that, from the geological nature of the rocks composing 

 the Holywood Hills being mostly Silurian slates, it would be 

 highly improbable that water in any volume would be dis- 

 covered, although a small accumulation from surface waters 

 might be found, but this would soon be exhausted. 



At the conclusion the paper was criticised by Miss Ryan, 

 Rev. J. Shiels, Professor Gregg Wilson, and Messrs. Robert 



