89 



and the North of Italy j in regard to the history of which, M. Von 

 Buch has supposed that what is now dolomite, was in the first instance 

 ordinary limestone ; the magnesia which they contain at present 

 having been absorbed from pyroxenic lava, by the forcible intrusion 

 of which both this and the contiguous rocks were elevated, dislocated 

 and contorted. The author assents to this theory ; and as the phe- 

 nomena of the tract described by M. Von Buch agree with those of 

 the vicinity of Nice, he ascribes the interchange of magnesian and 

 non-magnesian limestones, and the violent disturbances which both 

 have undergone in the latter instance, to the same cause which 

 M. Von Buch adduces, viz. the proximity of pyroxenic lava. Trap- 

 rocks, however, have not been observed very near Nice : but there 

 may be such, he conceives, 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 1'Estrelles, and by 

 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 ; and extreme nicety of discrimination in- 

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

 struct him. 



Dec. 5. — -The reading was begun, 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. 



Dec. 1 6. — Messrs. Lyell and Murchison's Paper, begun at the last 

 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 Montlosier, and the illustrations of that district 

 recently published by Mr. Scrope, conceive that what they have 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 peculiarities are 

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

 Auvergne j 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 



