240 Geological Society : — 



Bonney and Dr. Hicks concerning the area in question and especi- 

 ally as to the presence or absence of Precambrian rocks, the author 

 gave an account of his own explorations and their results, the 

 principal of which were the following. 



In the Bangor and Caernarvon area three distinct conglomerat.03 

 had been confounded. The only one that showed distinct uncon- 

 formity on the underlying rock was of Arcnig (Ordovician) age. 

 The rocks of the southern and central portion of the area were 

 essentially of igneous origin and might bo distinguished into two 

 groups, the southern probably intrusive, the northern certainly 

 eruptive. There is no evidence to show what interval of time elapsed 

 between the production of these two groups, nor which of them is 

 the earlier, although the author regards it as more probable that 

 the southern mass is of the earlier date and overlain by the northern 

 portion. The Bangor beds are derived from the denudation of the 

 volcanic series, and of rocks which may have been associated with 

 it, and they contain a series of conformable conglomerates of which 

 the great conglomerates near Bangor are members. They are the 

 continuation of the Cambrian rocks seen to the cast, and have not 

 undergone any serious alteration. The porphyries of Llyn Padarn 

 and Moel Tryfaen are contemporaneous lava-flows in the midst of 

 the Cambrian series, the overlying conglomerates being derived 

 from them and from the sedimentary Cambrian rocks to the west ; 

 and hence there is no certain proof of there being any Precambrian 

 rocks in the whole district, though it is probable that the rock 

 near Caernarvon belongs to an epoch distinct from and anterior to 

 the Cambrian. 



Jan. 11, 1888.— Prof. J. W. Judd, F.R.S., President, in the Chair. 



The following communications were read : — 



1. " On the Law that governs the Action of Plowing Streams." 

 By R. D. Oldham, Esq., F.G.S. 



The author, after describing how his attention was drawn to 

 the subject, proceeded to an investigation of the law that governs 

 the action of a flowing stream. Having accepted as a fundamental 

 principle that the velocity of a stream will always tend to become 

 such as is just sufficient to transport the solid burden cast on to the 

 stream, and pointed out that the principle is almost axiomatic in its 

 nature, he finds that, where untrammelled by exterior conditions, a 

 stream will be alternately confined to a single, well-defined, deep 

 channel, and spread out into a number of ill-defined, shallow 

 channels, the former being defined as a " reach," the latter as a 

 " fan," that the gradient in the " reach " is less than on the " fan," 

 and that both " reach " and " fan " will continually be encroaching at 

 their upper ends, and being encroached upon at their lower ends. 



After detailing some general considerations which show that 

 what should occur according to hypothesis, does actually occur in 

 nature, he indicated that the accurate and detailed levels taken in 

 connexion with the Ganges canal, do actually show this alternation 

 of " reach " and " fan," that the gradients are higher in tho latter, 



