202 LACUSTEINE STEATA — AUVERGNE. [Cn. XV 



4. Gypseous marls. — More than 50 feet of thinly laminated gypseoii.-^ 

 marls, exactly resembling those in the hill of Montmartre, at Paris, are 

 worked for gypsum at St. Romain, on the right bank of the AUier. They 

 rest on a series of green cypridiferous marls which alternate Avith grit, the 

 united thiclaiess of this inferior group being seen, in a vertical section on 

 the banks of the river, to exceed 250 feet. 



General arrangement, origin, and age of the freshwater formations 

 of Auvergne. — The relations of the different groups above described can- 

 not be learnt by the study of any one section ; and the geologist who 

 sets out with the expectation of finding a fixed order of succession may 

 perhaps complain that the diff'erent parts of the basin give contradictoiy 

 results. The arenaceous division, the marls, and the limestone, may ah 

 be seen in some places to alternate with each other ; yet it can by no 

 means be affirmed that there is no order of arrangement. The sands, 

 sandstone, and conglomerate constitute iu general a littoral group ; the 

 foliated white and green marls, a contemporaneous central deposit ; and 

 the limestone is for the most part subordinate to the newer portions of 

 both. The uppermost marls and sands are more calcareous than the 

 lower ; and we never meet with calc?>reous rocks covered by a consider- 

 able thickness of quartzose sand or green marl. From the resemblance 

 of the limestones to the Italian travertins, we may conclude that they 

 were derived from the waters of mineral springs, — such springs as even 

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

 granite, and j^recipitating 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, Avhen 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 transj^orted 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 



