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On the Excavation of Valleys, as illustrated by the Volcanic 

 Rocks of Central France. By Charles Lyell, Esq. 

 F. R. S., Vice-President of the Geological Society of Lon- 

 don, &c. ; and Roderick Impey Murchison, Esq. F. R. S. 

 Secretary to the Geological Society of London, &c. (Com- 

 municated by the Authors *.) 



A he illustration afforded by the volcanic phenomena of Cen- 

 tral France, of the power of rivers in excavating valleys, has 

 already engaged the attention of many authors ; but every ad- 

 ditional fact and argument bearing on this subject, will, it is 

 hoped, be of interest ; and apology will be unnecessary, should 

 we repeat, for the sake of perspicuity, some doctrines on the 

 origin of valleys, long ago announced by Saussure, Playfair, 

 and Montlosier, since it is well known that the opposite opi- 

 nions of De Luc and his school have enjoyed almost an equal 

 share of favour amongst our countrymen. 



Before we advert to examples of lava currents, which have 

 suffered considerable destruction from the erosive action of run- 

 ning-water, it will be necessary to remark some characteristic 

 peculiarities of form in such currents as have remained nearly 

 entire and unaltered from the time of their first flowing, either 

 because they descended over a surface elevated above the levels 

 of torrents or rivers, or because, where they occupied a water- 

 course, they diverted the stream into another direction, so that 

 its ne«r passage was not worked out through the lava. In these 

 cases, the lateral termination of the current is generally abrupt 

 and broken, and such as would not have been anticipated by 

 those who have not observed phenomena of like nature in lavas 

 proceeding from existing volcanoes. Thus the current, or 

 " Cheire -f-" of Puy de la Vache in Auvergne, crosses an in- 

 clined plane immediately on leaving its crater, and proceeds in 



" Read before the Geological Society of London, December 5th and 

 19th 1828. 



+ " Cheire" or " Cherre" is the local name of basaltic currents in Au- 

 vergne, and M. de Montlosier states (y>. 48. Essai stir la Theorie des Vokanos), 

 that it is derived from the Latin word " Seirae" signifying rugged places or 

 defiles; and hence, in the old deeds relating to this country, the word 

 " Cherulae." (Due.) 



