Cn. XIV.] LOWER MIOCENE OF CENTRAL FRANCE. 223 



miles ; and it is for the most part composed of nearly horizontal strata of 

 sand, sandstone, calcareous marl, clay, and limestone, none of which ob- 

 serve a fixed and invariable order of superposition. The ancient borders 

 of the lake, wherein the freshwater strata were accumulated, may gen- 

 erally be traced with precision, the granite and other ancient rocks rising 

 up boldly from the level country. The actual junction, however, of the 

 lacustrine and granitic beds is rarely seen, as a small valley usually in- 

 tervenes between them. The freshwater strata may sometimes be seen 

 to retain their horizontally within a very slight distance of the border- 

 rocks, while in some places they are inclined, and in few instances vertical. 

 The principal divisions into which the lacustrine series may be separated 

 are the following : — 1st, Sandstone, grit, and conglomerate, including red 

 marl and red sandstone. 2dly, Green and white foliated marls. 3dly, 

 Limestone or travertin, often oolitic. 4thly, Gypseous marls. 



1. a. Sandstone and conglomerate. — Strata of sand and gravel, some- 

 times bound together into a solid rock, are found in great abundance 

 around the confines of the lacustrine basin, containing, in different places, 

 pebbles of all the ancient rocks of the adjoining elevated country ; namely, 

 granite, gneiss, mica-schist, clay-slate, porphyry, and others, but without 

 any intermixture of basaltic or other tertiary volcanic rocks. These strata 

 do not form one continuous band around the margin of the basin, being 

 rather disposed like the independent deltas which grow at the mouths of 

 torrents along the borders of existing lakes. 



At Chamalieres, near Clermont, we have an example of one of these 

 deltas, or littoral deposits, of local extent, where the pebbly beds slope 

 away from the granite, as if they had formed a talus beneath the waters 

 of the lake near the steep shore. A section of about 50 feet in vertical 

 height has been laid open by a torrent, and the pebbles are seen to con- 

 sist throughout of rounded and angular fragments of granite, quartz, 

 primary slate, and red sandstone. Partial layers of lignite and pieces of 

 wood are found in these beds. 



At some localities on the margin of the basin quartzose grits are found ; 

 and, where these rest on granite, they are sometimes formed of separate 

 crystals of quartz, mica, and felspar, derived from the disintegrated granite, 

 the crystals having been subsequently bound together by a siliceous ce- 

 ment. In these cases the granite seems regenerated in a new and more 

 solid form ; and so gradual a passage takes place between the rock of 

 crystalline and that of mechanical origin, that we can scarcely distinguish 

 where one ends and the other begins. 



In the hills called the Puy de Jussat and La Roche, we have the advan- 

 tage of seeing a section continuously exposed for about 'ZOO feet in thick- 

 ness. At the bottom are foliated marls, white and green, about 400 feet 

 thick ; and above, resting on the marls, are the quartzose grits, cemented 

 by calcareous matter, which is sometimes so abundant as to form imbed- 

 ded nodules. These sometimes constitute spheroidal concretions 6 feet in 

 diameter, and pass into beds of solid limestone, resembling the Italian 

 travertins, or the deposits of mineral springs. 



