Laganne — Erosion of the Vezhre. 371 



ITI. — A. Laganne. — Note sur les JKrosions des Calcaires Denudes 

 DE la Vallee de la Vezere et de ses Affluents. Ann. 

 d'Agric, ScL, et Arts, Dordogne. (? 18G8.) (Pp. 8.) 



THIS short paper treats of the origin of the many channels that 

 furrow horizontally the limestones of the Valley of the Yezere, 

 etc. These channels have been thought to be the result of old cur- 

 rent-action, but M. Laganne shows that they have been formed by 

 atmospheric action, thereby confirming the opinion of M. Lartet, 

 and he thinks that frost is the principal, if not the only, agent 

 employed. 



The beds are mostly flat, but sometimes dip in the opposite direc- 

 tion to the flow of the neighbouring stream. In this latter case, if 

 there is a bed susceptible of attack from atmospheric agents, the 

 erosion of the channel follows that bed, and is therefore also in an 

 opposite direction to the stream, whereas if the channel had been 

 caused by current-action, it should clearly slope with the water-flow. 



The weathering away of underlying yielding beds has often caused 

 the fall of large masses of overlying firmer rock, and sometimes in 

 places where currents could not possibly have caused such falls. 



A means of measuring the rate of atmospheric denudation in the 

 district is given by the occurrence of holes cut in the rock for the 

 support of the rafters of old buildings of known age (about 1435), 

 which were destroyed, exposing the rock in which the holes are cut 

 to be acted on by the weather. Some of these holes are in a rock 

 that does not yield to frost, and therefore are well preserved ; others 

 however, were cut in rock that weathers away more or less readily, 

 and these have either wholly or partly disappeared. Assuming that 

 all were once of about the same depth, as is most likely to have been 

 the case, the rate of weathering is calculated to have been about 

 3-5ths of an inch in 20 years. This is a minimum for these rocks, as 

 atmospheric agents act slowly on the bed in which the above obser- 

 vation was made. 



The debris of the limestones form a talus sometimes many metres 

 thick, which, on the current-theory, should have been carried away. 



That the channels have not been caused by the chemical action of 

 water, the author thinks is proved by the fact, that no beds of a kind 

 that would result from such decomposition occur in the district. 

 Moreover, chemical action would, he thinks, destroy the debris quicker 

 than the solid rock. [Perhaps M. Laganne undervalues the chemical 

 action of carbonated water, which surely must have had a large share 

 in the wearing away of these limestones]. 



Lastly, the channels in question occur at all levels, and it is hard 

 to suppose that the waters have been lowered, from the highest levels 

 of the hills to their present level, so insensibly at to leave like traces 

 on all parts of their downward course. W. W. 



