228 
EOCENE PERIOD. 
[Ch. XVII. 
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 testaeea 
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. xiv. p. 249 ; and 2nd. Ed, p. 28G. 
