€h. XIV.] FRESHWATER FORMATIONS OF AUVERGNE. 227 



that ten or twelve tubes are packed within the compass of a cubic inch, 

 and that some single strata of this limestone are 6 feet thick, and may 

 be traced over a considerable area, we may form some idea of the count- 

 less number of insects and mollusca which contributed their integuments 

 and shells to compose this singularly constructed rock. It is unnecessa- 

 ry to suppose that the Phryganew lived on the spots where their cases 

 are now found ; they may have multiplied in the shallows near the 

 margin of the lake, or in the streams by which it was fed, and their 

 cases may have been drifted by a current far into the deep water. 



In the summer of 1837, when examining, in company with Dr. Beck, 

 a small lake near Copenhagen, I had an opportunity of witnessing a 

 beautiful exemplification of the manner in which the tubular cases of 

 Auvergne were probably accumulated. This lake, called the Fuure-Soe, 

 occurring in the interior of- Seeland, is about twenty English miles in 

 circurnference, and in some parts 200 feet in depth. Round the shallow 

 borders an abundant crop of reeds and rushes may be observed, covered 

 with the indusiae of the Phryganea grandis and other species, to which 

 shells are attached. The plants which support them are the bullrush, 

 Scirpus lacustris, and common reed, Arundo phragmites, but chiefly the 

 former. In summer, especially in the month of June, a violent gust of 

 wind sometimes causes a current by which these plants are torn up by 

 the roots, washed away, and floated off in long bands, more than a mile 

 in length, into deep water. The Cypris swarms in the same lake ; and 

 calcareous springs alone are wanting to form extensive beds of indusia] 

 limestone, like those of Auvergne. 



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

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

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

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

 united thickness 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 different parts of the basin give contradictory 

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

 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 in 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 calcareous 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 



