Ch. XVII.] LACUSTRINE STRATA PUY EN VELAY. 235 



lake. The transported materials must be arranged according 

 to their size and weight, the coarser near 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 



