228 KOCENE PERIOD. [ch. xvil. 



water 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 a few instances vertical. The 

 principal divisions into which the lacustrine series may be sepa- 

 rated are the following : 1st, Sandstone, grit, and conglomerate. 

 2ndly, green and white foliated marls. 3dly, limestone or tra- 

 vertin, oolite,, &c. 4thly, gypseous marls. 



1. Sandstone and conglomerate. Strata of sand and gravel, 

 sometimes bound together into a solid rock, are found in great 

 abundance around the confines of the lacustrine basin, contain- 

 ing, 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 the arenaceous 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 littoral groups of local extent where the pebbly beds 

 slope away from the granite as if they had formed a talus be- 

 neath 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 consist throughout of 

 rounded and angular fragments of granite, quartz, primary 

 slate, and red sandstone, but without any intermixture of those 

 volcanic rocks which now abound in the neighbourhood. Par- 

 tial layers of lignite and pieces of wood are found in these beds, 

 but no shells, a fact which probably indicates that testacea 

 could not live where the turbid waters of a stream were fre- 

 quently hurrying down uprooted trees, together with sand and 

 pebbles, or, that if they existed, they were triturated by the 

 transported rocks. 



There are other localities on the margin of the basin where 

 quartzose grits are found, composed of white sand bound 

 together by a siliceous cement. 



* See vol. i. chap. \\v. p. 2 19 ; and 2nd. Kd. p. 2,%. 



