i 50. FRESH-WATER STRATA OF AUVERGNE. 273 



of the beds contain bitumen ; others are entirely made up of the 

 cases of the Caddis-worm (Indusia tubulata), cemented together 

 by calcareo-siliceous matter. (This specimen, which was in the 

 cabinet of Faujas St. Fond, displays the characters of this re- 

 markable concrete : it consists of the tubes or cases of the larvae 

 of a species of phryganea, or may-fly.) These cases, which are 

 formed by the adhesion of shells to the outer surface of the silken 

 case secreted by the insect, are abandoned by the animal when 

 its metamorphosis is completed, and layers of them may often be 

 seen in our ditches and lakes. The fossil cases have been cemented 

 together by calcareous infiltration, and form a compact stone, 

 which is employed for building. The attached shells are so minute, 

 that often more than a hundred are affixed to a single case ; and 

 a cubic inch of the limestone includes ten or twelve tubes. If, 

 says Mr. Scrope, we consider that repeated strata, of five or six 

 feet in thickness, almost entirely composed of these fossils, once 

 extended over the whole plain of the Limagne, occupying a 

 surface of many hundred square miles, we may have some idea 

 of the countless myriads of minute beings which lived and died 

 within the bosom of that ancient lake. 



In the limestones, associated with land and fresh-water shells, 

 and remains of terrestrial plants, are bones of species of palaeo- 

 therium, anoplotherium, lagomys, marten, dog, rat, tortoise, 

 crocodile, serpent, and birds ; the lava currents that have 

 flowed over the strata have produced but little change in the 

 organic remains. This series comprises beds of gypseous and 

 laminated marls, with intercalations of siliceous limestone, con- 

 taining impressions of lacustrine and river shells. In some 

 localities, the fresh-water limestone has an intermixture of 

 volcanic matter, and presents the characters of a sediment 

 slowly and tranquilly deposited in a lake, into which ashes, 

 and fragments of rocks and scoriae, were projected by a neigh- 

 bouring volcano ; while some beds appear to have been formed 

 by a violent intrusion of volcanic products. 



3d. Immense beds of basalt, scoriae, &c, spread over the tabular 

 masses of fresh-water limestone, and often capping the summits 

 of the lower hills. (PI. II. fig. m.) 



4th. Sand and diluvial gravel, containing bones of the mastodon, 

 elephant, hippopotamus, rhinoceros, tapir, horse, boar, hyena, 

 bear, dog, beaver, hare, kc. : with these are associated lignite, 

 and other vegetable remains. Some of the beds of limestone 

 abound in seed-vessels of charae ; and the laminated clays con- 

 tain fishes, with leaves and stems of reeds and other plants. 

 T 



