Reports & Proceedings — Geological Society of London. 187 



As an example, tlie gorge of the E,iver Alyn ia Denbighsliire was 

 quoted. The post-Glacial excavation was of measurable size, and 

 its contents had been sliot on to a plain in the form of a fan of 

 measurable extent. Fans spi'ead on the flat bottoms of valleys by 

 tributary streams, or deltas formed in lakes, were of common occur- 

 rence. In all cases it would be of value to determine a relation 

 between three factors, namely, the size of the fan or delta, the 

 discharge of the stream, and the character of the ground from which 

 the material was derived. In one case, in the Vale of Neath, the 

 stream was carried over a railway on an aqueduct, and the amount 

 added annually by it to the fan could be observed in its passage. 



Lakes occupying hollows among Drift-mounds gave a different 

 problem. The material accumulating in them was blown and 

 washed by rain from the surrounding slopes. 



Dammed-up rivers gave opportunities for observing the amount of 

 material transported by rolling. Canalized rivers were not paralysed 

 as denuding agents, but asserted themselves in time of flood. The 

 amounts dredged at various points on the canalized channel should 

 give an accurate measure of the activities of the river. 



The distance over which rivers were now transporting material 

 should be ascertainable by observing the composition of recent alluvial 

 deposits. Few rolled gravel directly into the sea, for the gradients 

 in the lower parts of their courses were too low for transportation and 

 favoured deposition. A diagram of the Exe, Medway, and Severn, 

 showing their gradients from source to mouth, illustrated this and 

 brought to light interesting differences. The last two had long tidal 

 reaches and perfectly-graded middle reaches, with sharp upturns 

 to the source which were common to all rivers. The Exe had 

 a comparatively short tidal reach and a more steeply and less 

 perfectly-graded middle reach. A change in the gradient was 

 observable where the Exe passed from Devonian rocks on to Culm 

 Measures. 



An investigation on English rivers had been proceeding for some 

 years with the object of ascertaining (1) the discharge, (2) the 

 suspended and dissolved impurities, (3) the rainfall, (4) the areas 

 of the basins, and (5) the character of the rocks. 



The rivers suitable for the investigation were limited to those with 

 a single definite mouth. Fen-rivers had a number of outlets, and 

 could not be gauged. The Exe, Medway, and Severn were selected 

 as examples of an upland stream and of canalized lowland rivers. 

 It was sought to establish a definite relation between the level of the 

 water on the one hand and tlie velocity of the current and the amount 

 of suspended or dissolved matter on the other hand. The results were 

 partly in accordance with anticipation. The suspended matter was 

 scarcely measurable until the water reached a certain level above 

 normal, but increased rapidly for higher levels. The dissolved 

 matter was at its maximum when the river was lowest and 

 diminished as the level rose. 



The amount of material now being rolled by the Exe was deter- 

 mined from records of dredgings. Rainfall was dealt with by the 

 British Rainfall Organization, as also the methods of eliminating 



