384 NATURAL SCIENCE. June, 1S95. 



geologists, and I may conclude with an apt quotation from a recent 

 French work on Earth-sculpture 3, the authors of which write as follows 

 (p. 170) : — " The deep valleys of the causses would not have existed 

 if their drainage basins had been limited to the permeable limestone 

 areas. The rain-water would have sunk into the ground instead of 

 forming rivers large enough to excavate such deep ravines. On the 

 contrary, the watercourses formed on the impervious rocks . . . 

 entered the permeable region with a sufficient volume to erode their 

 channels . . . The rocks in the upper reaches of the valleys 

 were crumbled and washed down by the action of the rain ; while the 

 limestones which crown the causses, thanks to their permeability, 

 escaped this detritive action. This is sufficient to explain the aspect 

 of the region round Florae, where the Tarn and its affluents seem to 

 penetrate the thickness of the plateau as if they passed through a 

 wall " ; meaning, of course, that the outer wall of the plateau has 

 been developed pari passu with the excavation of the valleys, and was 

 not there when the streams commenced their work. 



For the inception of this work we must look back to early 

 Tertiary times (Eocene or Oligocene), when the country was being 

 elevated after the great Cretaceous submergence and when the 

 Cretaceous and Jurassic limestones formed an extensive plain sloping 

 gently away from the central highlands of France. It was then that 

 the drainage system of the central provinces was marked out ; the 

 decomposition of the granites, gneisses, and lavas of Auvergne and 

 Limousin supplied plenty of grit for erosive purposes, and the rivers, 

 having once established their channels, slowly sawed their way down 

 through the layers of limestone. The upward movement of the land, 

 whether this was continuous or episodic, kept the machinery in 

 action and enabled the rivers to eat deeper and deeper into the tracts 

 which subsequently became, by isolation, the elevated plateaux or 

 causses of modern times. 



Such I conceive to be the history of the formation of these 

 remarkable valleys, and all the other physical features of the region 

 are really dependent on the gradual formation of these deep ravines. 

 The caverns which open at various levels in their walls register 

 stages in the process of erosion, for the highest caves mark the time 

 when the water issued at that level ; but as the canons were cut 

 deeper and deeper the subterranean waters were drawn off at lower 

 and lower levels, either deserting their former tunnels or deepening 

 them in places into underground canons. Mr. Martel has explored 

 many of these gloomy corridors and found them open here and there 

 into lofty halls curtained by a profusion of stalactitic ornaments, like 

 the grottoes of Adelsberg and other well-known subterranean palaces. 4 



A. J. Jukes-Browne. 



3 " Les Formes du Terrain," par MM. de la N6e et de Margerie. Paris, 18S8. 



4 See " Les Abimes," reviewed in Natural Science, vol. vi., p. 131. Feb., 1895. 



