94 Reports and Proceedings— 
voleanic 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 east, 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. 
IJ.—January 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 Flowing 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 in 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 the latter, 
as they should be, and that the records of the Canal show the retro- 
gression of ‘“ fan” and “reach” demanded by the hypothesis. 
Accepting this agreement of fact with hypothesis as proof of the 
correctness of the latter, it follows that the fundamental principle 
on which it is founded is correct, and that, in the absence of inter- 
fering causes of greater potency, it is the coarseness or fineness of 
the débris cast upon-a stream that will determine its gradient and 
velocity, and not, as stated in text-books, the velocity of a stream 
that will determine its gradient and the coarseness of the débris 
transported by it:—a conclusion that might be arrived at inde- 
pendently, from the fact that it is in the upper reaches of a stream, 
where coarse débris prevails, that high velocities of current prevail, 
while in the lower reaches, where the débris is finer in grain, the 
velocity of current is also diminished. 
2. ‘Supplementary Notes on the Stratigraphy of the Bagshot 
