Ch. XVII.] LACUSTRINE STRATA PUY EN VELAY. 
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lake. The transported materials must be arranged according 
to their size and weight, the coarser Hear the shore, the finer 
at a greater distance from land ; but in the gravelly and 
sandy beds of Lake Superior no pebbles of modern volcanic 
rocks can be included, since there are none of these at present 
in the district. If the igneous action should break out in that 
country and produce lava, scoriae, and thermal springs, the 
deposition of gravel, sand, and marl, might still continue as 
before ; but in addition, there would then be an intermixture 
of volcanic gravel and tuff, and rocks precipitated from the 
waters of mineral springs. 
Although the fresh-water strata of the Limagne approach 
generally to a horizontal position, the proofs of local disturb- 
ance are sufficiently numerous and violent to allow us to sup- 
pose great changes of level since the Eocene period. We are 
unable to assign a northern barrier to the ancient lake, although 
we can still trace its limits to the east, west, and south, where 
they were formed of bold granitic eminences. But we need 
not be surprised at our inability to restore the physical geo- 
graphy of the country after so great a series of volcanic erup- 
tions. It is by no means improbable that one part of the 
district may have been moved upwards bodily, while the others 
remained at rest, or even suffered a movement of depression. 
Puy en Velay. — In the department of the Haute Loire, a 
fresh-water formation, very analogous to that of Auvergne, is 
situated in the basin of the Loire, and is exposed in the valley 
in which stands the town of Le Puy. Since the deposition of 
the lacustrine strata, there have been so many volcanic erup- 
tions in this country, and such immense quantities of lava and 
scoriae poured out upon the surface, that the aqueous rocks are 
almost buried and concealed. We are indebted, however, to 
the researches of M. Bertrand de Doue for having distinctly 
ascertained the succession of strata, and we have had opportu- 
nities of verifying his observations during a visit to Le Puy. 
In this basin we find, as in Auvergne, two great divisions, 
consisting of grits and marls ; the former composed of quartzose 
