224 LACUSTRINE STRATA, AUVERGNE. [Ch. XIV. 



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

 arenaceous groups is one of red sandstone and red marl, which are iden- 

 tical in all their mineral characters with the secondary New 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- 

 vergne, 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 

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

 amassed near the northern shores, so in Auvergne the grits and con- 

 glomerates before mentioned were evidently formed near the borders. 



The entire thickness of these marls is unknown ; but it certainly ex- 

 ceeds, in some places, 700 feet. They are, for the most part, either light- 

 green or white, and usually calcareous. They are thinly foliated, — a 

 character which frequently arises from the innumerable thin shells, or 

 carapace-valves, of that small crustacean called Cypris, which is pro- 

 vided with two small valves, not unlike those of a bivalve shell, and 

 moults its integuments periodically, which the conchiferous mollusks do 

 not. This circumstance may partly explain the countless myriads of the 

 shells of Cypris which were shed in the ancient lakes of Auvergne, so as 

 to give rise to divisions in the marl as thin as paper, and that, too, in 

 stratified masses several hundred feet thick. A more convincing proof of 

 the tranquillity and clearness of the waters, and of the- slow and gradual 

 process by which the lake was filled up with fine mud, cannot be desired. 

 But we may easily suppose that, while this fine sediment was thrown 

 down in the deep and central parts of the basin, gravel, sand, and rocky 

 fragments were hurried into the lake, and deposited near the shore, form- 

 ing the group described in the preceding section. 



