198 UPPER EOCENE PERIOD. [Ch. XV 



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 '700 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 sohd limestone, resembliog the Itahan 

 travertins, or the deposits of mineral springs. 



1. b. Bed marl and sandstone. — But the most remarkable of the 

 arenaceous groups is one of red sandstone and red marl, w^hich are iden- 

 tical in all their mineral characters with the secondary Neiv Red sand- 

 stone and marl of England. In these secondary rocks the red ground is 

 sometimes variegated with light greenish spots, and the same may be 

 seen in the tertiary formation of freshwater origin at Coudes, on the Al- 

 lier. The marls are sometimes of a purplish-red color, as at Champheix, 

 and are accompanied by a reddish limestone, like the well-known " corn- 

 stone," which is associated with the Old Red sandstone of English geol- 

 ogists. The red sandstone and marl of Auvergne have evidently been 

 derived from the degradation of gneiss and mica-schist, which are seen 

 in situ on the adjoining hills, decomposing into a soil very similar to the 

 tertiary red sand and marl. We also find pebbles of gneiss, mica-schist, 

 and quartz in the coarser sandstones of this group, clearly pointing to 

 the parent rocks from which the sand and marl are derived. The red 

 beds, although destitute themselves of organic remains, pass upwards 

 into strata containing tertiary fossils, and are certainly an integral part of 

 the lacustrine formation. From this example the student will learn how 

 small is the value of mineral character alone, as a test of the relative age 

 of rocks. 



2. Green and white foliated marls. — The same primary rocks of Au- 

 vergoe, which, by the partial degradation of their harder parts, gave rise 

 to the quartzose grits and conglomerates before mentioned, would, by the 

 reduction of the same materials into powder, and by the decomposition 

 of their felspar, mica, and hornblende, produce aluminous clay, and, if a 

 sufficient quantity of carbonate of lime was present, calcareous marl. 

 This fine sediment would naturally be carried out to a greater distance 

 from the shore, as are the various finer marls now deposited in Lake 

 Buperior. And as, in the American lake, shingle and sand are annually 



