102 M. Brongniart on the Fresh-water Formations of Italy. 
of this work, I might cite examples of thermal springs still con- 
taining, in the present state of the surface of the globe, a great 
quantity of silex in solution. 
The freshwater formations of CEningen have a character al- 
together different from those of Italy, of the neighbourhood of 
Paris, and of the greater number of those which I have men- 
tioned, and on this account a different origin must be attri- 
buted to them. They may be regarded as sedimentary fresh- 
water formations, produced almost entirely by mechanical means, 
that is to say, by earthy matters, often even pretty coarse, de- 
posited more or less rapidly at the bottom of a lake, and enve- 
loping the organised bodies which may occur there. They also 
present a structure entirely different from the others : the layers 
are very distinct and numerous ; the grain is sandy, and often 
coarse ; lastly, there are none of those tubular perforations, 
which indicate in the other formations the disengagement of the 
gas which the limestone holds in solution. 
We have therefore two sorts of fresh-water formation^, very 
different in their origin, and distinguishable by external charac- 
ters, which indicate this difference of origin. The one, produ- 
ced by solution and precipitation, more or less pure and crystal- 
line, have issued from the interior of the earth with the waters 
which have carried them to the surface of the soil. They may, 
according to this theory, be formed at all elevations where simi- 
lar waters may have issued, and the height at which they occur 
is not always a proof of that to which the fresh-water has been 
elevated. These are the most extensive : they are those of the 
neighbourhood of Paris, of Locle, of Italy, &c. ; they are rare- 
ly mixed with bodies of marine origin. 
The others, of coarser texture, resulting, so to speak, from 
the abrasion and washing of the surface of the rock, are 
formed by means of sediment at the bottom of still waters, into 
which they have been carried. They are much less diffused, 
less pure, and may contain remains of marine bodies : of this 
kind are the CEningen formation, a part of the Limagne of 
Auvergne, and probably the plastic clays and lignites. Lastly, 
it is to this class that the beds of fresh-water formation belong, 
which we observe in the Mollasse of Switzerland*. 
* From the new edition of Cuvier’s work ou Organic Remains, about to be pub- 
lished. 
