in its relation to Agriculture. 
433 
both at Paris and Kouen. AltlK)up;h the precipitation of the 
carbonate of lime is easily explained, the question still remains, 
how is the silicic acid so acted upon that it underfjoes a dimi- 
nution from r4t56 to 1-157 -grains per litre (= 1"77 pint)? It 
has been sug^sjested that the carljonic acid disengaging^ itself from 
carbonate of lime?, or being raj)idly generated in a water rich in 
oxygen and reccnving a copious supply of decomposed organic 
matter, may act upon the various silicates so as to dissolve them. 
I merely allude to these questions to show of what importance 
they are in regard to the application of the waters of our streams 
for the purpose of irrigation. 
There are, however, in France many disturbing causes, such 
as violent rain-storms, which suddenly and in a great degree 
alter the chemical character of the waters of the large rivers. 
The Loire, for instance, after a great fall of rain, becomes strongly 
impregnated with silicic acid and silicates, because its own 
source and that of most of its affluents take their origin and flow 
for a considerable distance through feldspathic strata. On the 
other hand, it is important to observe that rivers, when flowing 
through marshy valleys, frequently get their waters poisoned 
by noxious solutions, adverse to plants of a higher order. Their 
sluggish waters, loitering in the midst of a mass of rank aquatic 
vegetation, are robbed of nearly the whole of their oxygen by 
the requirements of these greedy plants ; and they moreover 
dissolve decomposed organic debris, which tinge them with a 
dark opaque hue, and impart to them an offensive and unhealthy 
odour. Such are several of the affluents of the lower Loire. 
Having thus incidentally noticed the principal chemical 
features of the Loire, I will rapidly examine the other important 
geological and chemical features of the Gironde and the Rhone, 
deeply regretting that the limits of this paper do not permit me 
to follow the interesting researches made on many of the tribu- 
taries of the Loire by French chemists, especially MM. Bobierre 
and Merode, and record the very remarkable phenomena they 
have noticed and examined, and the valuable mass of informa- 
tion they have collected. The following are the chief geological 
features of the Loire basin: — From its source to its junction 
with the Allier, the Loire flows over a series of crystalline 
formations, such as granite, gneiss, porphyry, and volcanic rocks. 
From its confluence with the Allier, that is from Nevers to 
Angers, it traverses the skirt of secondary strata, which surround 
the great northern tertiary basin. At Orleans its basin is only 
divided from that of the Seine (with which it is made to com- 
municate by the canal of Briare) by a very slight undulation in 
the ground, of so easy a declivity that the locomotives of the rail- 
way ascend it with the greatest ease. This explains why the 
