GEOLOGY OF THE VEZERE. 35 



Quatritme Etage. The fourth group of Cretaceous limestones seems to be absent in the course of the 

 Vezere ; but its uppermost Beds (No. 1 of the following list) appear between the Perigucux and Thmers 

 Road and Millac-de-Nontron, lying between the limestones of the " Third Stage " and the " Upper Oolite ; " 

 and they probably extend to the S.E. They have been well seen around Mareuil. To complete our 

 account of the Cretaceous series of the Dordogne we briefly note that this " Fourth Stage " consists of: 



1. Limestones, varying from white to yellow, from friable to compact, and sometimes sandy and glauconitic, 



and often abounding with Alveolince, Caprince, Eadiolites, Echinoderms, and other fossils. M. d* Archiac's 

 " Lower Rudistes-zone." 



2. Grey, green, and ferruginous sands and sandstones, more or less calciferous. 



3. Yellow or greyish, marly, cellular limestone, and calciferous concretionary sandstones. The limestone 



contains Alveolince, Orbitoides, Corals, Rudistes, Echinoderms, and other fossils. 



4. Greyish pyritr us clays, with lignite. (These sometimes alternate with the beds No. 3.) 



The Caves. The ossiferous caves and recesses (whether or not, in some cases, 

 enlarged artificially) have been hollowed out, as already noticed, by atmospheric 

 agency, where the softer alternate with the harder bands of limestones, the latter 

 often still forming more or less continuous ledges. The effect of the last winter's 

 frosts in flaking off the concave faces of the softer beds is often conspicuous along 

 these cliffs. 



In the Cave qf Le Moustier* there is much red, sandy, micaceous alluvium, 

 very similar to the brick-earth of the valley below. It is not necessary, however, to 

 suppose that the cave was on a level with the flood- waters of the valley since Man 

 inhabited it; for, as Mr. J. Evans has suggested t, the sand may either have been 

 blown in by the winds, or, possibly, it may have reached the cave from the top of 

 the hill during the formation of a talus, removed for the most part since that time 

 by the river having swept the foot of the cliff, from which it has now receded. 



The little valley in which the Cave called Gorge d'Enfer occurs shows (as 

 Mr. Evans pointed out J) how the talus of the cliffs themselves may have choked 

 the caves, thus accounting for some of the recesses being filled up to the roof. On 

 one side of this valley the edges of the limestone strata are covered with a grass- 

 grown talus ; on the other the cliff is prominent, bare, and cavernous (see "Wood- 

 cut, fig. 2, page 4). The alluvial bottom of the valley being cultivated as a " water- 

 meadow," the old stream has been diverted to a small channel immediately under 

 the bare prominent cliff, and made to distribute itself thence in rectangular rills 

 at intervals over the soil. Whatever amount of talus may have accumulated 

 on what is now the bare side, the above-mentioned process of cultivation has 

 probably succeeded in removing, down to a line of ledges and recesses level with 



* See "Woodcut, fig. 1, page 3 ; and Lithographic Sketch, No. 1. 



t In a Paper read before the Geological Society, June 22, 1864. In the same Memoir. 



