Correspondence—Mr. G. HE. East—Rev. A. Irving. 239 
paper “ On the Forests of the Coal Period,” in which he remarks that 
the plants “grew in extensive level plains... . The moist atmo- 
sphere (not at all likely to have been charged with more carbonic acid gas 
than that of our own day)’ would encourage the growth of cellular 
parasites, ete.” The second reference is to a paper by Dr. H. Wood- 
ward, “ On Old Land Surfaces.” in which, after quoting some remarks 
by Dr. Sterry Hunt to the effect that the atmosphere of the Coal 
period contained, as originally suggested by Brongniart, a “ compara- 
tively large amount of carbonic acid,” he adds in a footnote, ‘“ Later 
experiments have, however, proved that plants, like animals, are at 
once poisoned by an excess of carbonic acid.” Now the first reference 
appears to me only the expression of an opinion, and in the second, 
although experiments are mentioned, the reference is not given. I 
know of no such experiments, and if your reviewer or any of your 
correspondents can refer me to any, I shall feel very much obliged. 
The only experiments bearing on the subject, and which show that 
plants can live, flourish, and grow rapidly in an atmosphere with an 
excess of carbonic acid, I have quoted (p. 120), and I know of no 
others. Excuse the length of this letter, but I am anxious for infor- 
mation on this point, and should be glad of confirmation or other- 
wise on this subject, which is one of much theoretical interest. 
Darent-Huume, SHOREHAM, SEVENOAKS, JosepH PRESTWICH. 
10th April, 1888. 
SPURIOUS FLINT IMPLEMENTS. 
Srr,—Will you kindly allow me space in the Grox. Mag. to inform 
its readers who may be collectors of Flint Implements, that there are 
at the present time being manufactured in London worked flints, 
which are stated to be genuine, but which are nothing of the sort, 
and at the same time to say that some of these manufactured flints 
have been sold to gentlemen for a high price, who are considered 
authorities on the subject, and I trust that should any of my 
readers meet with such as appear doubtful they will use their best 
endeavours to expose and stop such a fraud. Gro. HE. Hasr. 
241, Everinc Roap, Uprrr Cuapron, E. 
ALPINE RIVERS AND BUNTER PEBBLES. 
Str,— Prof. Bonney’s paper on the ‘‘ Rounding of Alpine Pebbles ” 
is a valuable contribution to a chapter of physical geology; but 
there are one or two considerations to which I do not think he has 
given sufficient recognition. (1.) Weathering of débris on the 
mountain-sides, which often gives a certain initial rotundity to frag- 
ments of rocks. (2.) The scouring action of sand in a mountain 
river. So far as I can recall my own Alpine observations, I am 
inclined to think that where the coarser detritus is most completely 
rounded, so that the pebbly form is generally produced, it has been in 
cases where a large proportion of sandy detritus was present also. 
On the other hand, I have generally found that at the mouths of 
1 The Italics are mine. 
