Ixxxviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 1914, 



necessary to examine the raw and freshly-deepened channels of the 

 streams which have formed them, to realize that the mechanism by 

 which they have been spread is still in active operation. In one 

 case the proof is particularly interesting. A delta occupying about 

 an eighth of a square mile has been spread on to the alluvial flat 

 by a stream which drains a tract of mountain -land. The stream is 

 now carried across the Great Western Railway at Resolven Station 

 in a broad iron trough. In time of flood, gravel and large stones 

 are rolled in procession along the trough and, without some help 

 from the railway officials, would speedily block the passage. Here 

 there would seem to be an opportunity of ascertaining the amount 

 of material which is being added to the delta in any given time 

 from a known area by a stream of measurable flow. 



In numberless other cases throughout the British Isles it would 

 be of value for our present purpose to determine the relation 

 between the size of the delta and the discharge of the stream which 

 has formed it, for various types of geological formations. 



Of deltas formed under water there is no lack in the lakes of 

 the British Isles. Many varieties are presented, but the simplest 

 perhaps are those formed where a stream enters at the head 

 of a lake and escapes at the foot, dropping its solid contents and 

 levelling up the lake-depression systematically from the head 

 downwards. Others which have been pushed out from a fairly 

 straight and regular shore-line present opportunities of measurement. 

 Less suitable for our present purpose are those formed where the 

 exit of the stream is close to its entry, or where a large feeder has 

 thrown a bar across the foot of the lake, as has happened where 

 the Treweryn enters the depression in which lies Bala Lake. 



Other types of lakes are furnished by the innumerable meres 

 which occupy hollows among drift-mounds, notably in Cheshire and 

 Shropshire, but in many other parts of the country also. Few of 

 these are entered by large streams, and they are being filled for the 

 most part by rainwash from the neighbouring slopes and wind- 

 borne material from far and wide. The post-Grlacial sediments in 

 them are not easily distinguished from the material which forms 

 their original sides and would be difficult to measure, but it is 

 probable that in some cases an estimate of their bulk would be 

 possible. Measurements in such cases would furnish data bearing 

 on the action of rain and wind, as distinct from the rough and 

 ready methods of a river. 



Time is an unknown factor in all the cases to which I have 



