228 LACUSTRINE STRATA, AUYERGNE. [Ch. XIV. 



now exist* in Auvergne, and which may be seen rising up through the 

 granite, and precipitating travertin. They are sometimes thermal, but 

 this character is by no means constant. 



It seems that, when the ancient lake of the Limagne first began to be 

 filled with sediment, no volcanic action had yet produced lava and scoriae 

 on any part of the surface of Auvergne. No pebbles, therefore, of lava 

 were transported into the lake, — no fragments of volcanic rocks im- 

 bedded in the conglomerate. But at a later period, when a considerable 

 thickness of sandstone and marl had accumulated, eruptions broke out, 

 and lava and tuff were deposited, at some spots, alternately with the 

 lacustrine strata. It is not improbable that cold and thermal springs, 

 holding different mineral ingredients in solution, became more numerous 

 during the successive convulsions attending this development of volcanic 

 agency, and thus deposits of carbonate and sulphate of lime, silex, and 

 other minerals were produced. Hence these minerals predominate m 

 the uppermost strata. The subterranean movements may then have 

 continued, until they altered the relative levels of the country, and caused 

 the waters of the lakes to be drained off, and the farther accumulation 

 of regular freshwater strata to cease. 



We may easily conceive a similar series of events to give rise to anal- 

 ogous results in any modern basin, such as that of Lake Superior, for 

 example, where numerous rivers and torrents are carrying down the 

 detritus of a chain of mountains into the lake. The transported mate- 

 rials 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 igneous action should break out in that country, and pro- 

 duce lava, scorise, 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 of rocks precip- 

 itated from the waters of mineral springs. 



Although the freshwater strata of the Limagne approach generally to 

 a horizontal position, the proofs of local disturbance are sufficiently 

 numerous and violent to allow us to suppose great changes of level since 

 the lacustrine 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 granite eminences. Nor need 

 we be surprised at our inability to restore entirely the physical geography 

 of the country after so great a series of volcanic eruptions ; for it is by 

 no means improbable that one part of it, the southern, for example, may 

 have been moved upwards bodily, while others remained at rest, or even 

 suffered a movement of depression. 



• It is scarcely possible to determine the age of the oldest part of the 

 freshwater series of the Limagne, large masses both of the sandy and 

 marly strata being devoid of fossils. Some of the lowest beds may be 

 of upper Eocene date, although, according to M. Pomel, only one bone 



