136 Geological Society. 



have been elevated, dislocated and contorted. The author assents to 

 this theory ; and as the phaenomena of the tract described by M. Von 

 Buch agree with tliose of the tract described by himself, he ascribes 

 the interchange of magnesian and non-magnesian limestones, and 

 the violent disturbances which both have undergone in the vicinity 

 of Nice, to the same cause which M. Von Buch adduces, viz, the 

 proximity of pyroxenic lava. The trap-rocks have not been observed 

 very near Nice : there may be such, however, within a short] distance 

 in depth ; and the probability that there are, is strengthened by the 

 prevalence of rocks of this class in the mountains of S. Troper and 

 i'Estrelles, and the frequency of pebbles both of trap and porphyry 

 in the tertiary conglomerates above described. 



The occurrence of dolomite and gypsum in what the author consi- 

 ders as the oolite formation, and the impracticability of recognizing 

 in this formation near Nice any of the individual beds of which it is 

 composed in England, are new proofs of the danger of judging of 

 large tracts of country by rules derived from the study of detached 

 specimens. — The same stratum, in different parts of Europe, assumes 

 very different appearances j and extreme nicety of discrimination in- 

 judiciously applied^ is more apt to mislead the geologist than to in- 

 struct him. 



Dec. 5. — The reading of a paper " On the Excavation of Valleys, 

 as illustrated by the Volcanic Rocks of Central France," by Charles 

 Lyell, Esq. V.P.'G.S. F.R.S. &c. and R. I. Murchison, Esq. For. Sec. 

 G.S. F.R.S. &c. was begun. 



Dec. 16.— Messrs. Lyell and Murchison's paper, begun at the for- 

 mer Meeting, was concluded. 



The theory, long since enounced, which ascribes the excavation of 

 valleys to the long-continued erosion of streams, has been supposed 

 to derive remarkable support from the appearances of the volcanic 

 tracts in the interior of France j and the authors, referring especially 

 to the works of M. De Monlosier, and the illustrations of that district 

 recently published by Mr. Scrope, conceive that what they had seen 

 themselves in Auvergne and the Vivarrais, strongly confirms the views 

 of these and other preceding writers. 



1. In the commencement of this paper, several peciiliarities "are 

 stated in the original form of the lava-currents, or " cheires," of 

 Auvergne ; which, if overlooked, might lead to an exaggerated es- 

 timate of the quantity of matter removed by the action of rivers. The 

 abruptness, especially, of the lateral termination of many of these 

 currents, is very remarkable, even where the lavas flowed in open 

 spaces, and where the surface has remained entire and apparently 

 unaltered since the time of their consolidation. But the authors still 

 conceive that the waste exclusively attributable to running water 

 and its detritus in Central France, must in the course of ages have 

 exerted a most powerful influence on the external form of the country. 



2. In the new Valley, about 2.50 feet in depth, opened at the Etang 

 de Fung by the waters of the Sioule, after the stream had been di- 

 verted from its course by the lava of Come, the matter removed, 

 and still continually carried away by the river, consists of alluvial 

 clay and sand, — and in some cases of the subjacent gneiss, thus ex- 

 cavated 



