| OCR Text |
Show 228 [Ch. XVI!. water strata may sometimes be seen to retain their horizontality within a very slight distance of the border-rocks, while in some places they are inclined, and in a few in.stance~ vertical. The principal divisions into which the lacustrm: senes may be sepa~ rated are the following: lst, Sandstone, gnt, and conglomerate. 2ndly, green and white foliated marls. Bdly, limestone or travertin ' oolite, &c. 4thly, gypseous marls. . 1. Sandstone and conglomerate.-Strata of sand and grave], 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 ~ricient 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 th~· 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 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 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. Partial 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 frequently 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. • Sec ,·ol. i. chap. xiv. p. 2~9; nnd 2nd, :Ed. p. 28G. Ch. XVII.] LACUSTRINE STRATA-AlJVERGNE. 229 Occasionally, when the grits rest on granite, as at Chamalieres before mentione~, and many other places, the separate crys~als of quartz, m1ca, and felspar, of the disintegrated gramte, are bound together again by the silex, so that the granite seems regenerated in a new and even more solid form and thus so gradual a passage may sometimes be traced be-' tween a crystalline rock and one of mechanical origin, that we can scarcely distinguish where one ends and the other begins. In the Puy de J ussat, and the neighbouring hill of La Roche, are white quartzose grits, cemented into a sandstone by calcareous matter, which is sometimes so abundant as to form imbedded nodules. These sometimes constitute spheroidal concretions six feet in diameter, and pass into beds of solid limestone resembling the Italian travertins, or the deposits of mineral springs. In the hills above mentioned, we have the advantage of seeing a section continuously exposed for about 700 feet in thickness. At the bottom are foliated marls, white and green, about 400 feet thick, and above, resting on the marls, are the quartzose grits before mentioned with the associated travertins. This section is observed close to the confines of the basin, so that the Jake must here have been filled up near the shore with fine mud, before the coarse superincumbent sand was introduced. There are other cases where sand is seen below the marl. 2. Red marl and sandstone.-But the most remarkable of the arenaceous groups is a red sandstone and red marl, identical in all their characters with the secondary new 'red sandstone and mm:l of England. In the latter, the red ground is sometimes ~a~1egate~ with light greenish spots, and the same may be seen In 1ts tertmry counterpart of fresh-water ori()'in at Coudes on the Allier, 'f~e marls are sometimes of a pu~plish-red col~ur, ~sat Champhe1x, and are accompanied by a reddish limestone, hke the well-known 'cornstone,' which is associated with the old red sandstone of English geologists. The red sandstone and marl of Auvergne l1ave evidently been derived from the |
