140 



nous. Lastly, the author dissents entirely from the theory of Von 

 Buch that the dolomitic mountains of the Alps have derived their 

 magnesia from augite rocks in fusion, and their peaked forms from 

 a simultaneous alteration of their structure : 



1st. — Because no trap or augite rocks occur in this region. 



2d. — Because fossil fish and plants in bituminous schist alternate 

 with beds of the dolomite, which must therefore have been of contem- 

 poraneous origin. 



3d. — Because the peaked outline of these mountains is sufficiently 

 explained by the high inclination, vast dislocations, and numberless 

 contortions, of the strata. 



The reading of a paper, " On the tertiary deposits of the Cantal, 

 and their relation to the Primary and Volcanic Rocks ;" By C. Lyell, 

 Esq., For. Sec. G.S., F.R.S., &c. ; and Roderick Impey Murchison, 

 Esq., Sec. G.S., F.R.S., &c, was begun. 



May 1. — Samuel Cartwright, Esq., of 32 Old Burlington Street, 

 and John Hall, Esq., of Edinburgh, were elected Fellows of this So- 

 ciety. 



The reading of a paper " On the tertiary deposits of the Cantal, and 

 their relation to the primary and volcanic rocks/' by Charles Lyell, 

 Esq., For. Sec. G.S., F.R.S., &c, and R. I. Murchison, Esq., Sec. 

 G.S., F.R.S., &c. begun at the last meeting, was concluded. 



The authors have selected this district for description, because, al- 

 though the adjoining fresh-water formations of the Limagne d'Au- 

 vergne, and of Puy en Velay, have been largely written upon ; yet this 

 of the Cantal has scarcely been noticed by any geologists, except in a 

 cursory manner by Mr. Scrope, and formerly by M. Brongniart in his 

 general observations on fresh-water deposits. (Annates du Museum, 

 torn. xv. 1810.) 



The fresh-water formations of Aurillac, or the Cantal is not a con- 

 tinuous portion of the great lacustrine deposits of the Limagne d'Au- 

 vergne, from which it is distinctly separated, being bounded on the 

 north, west and south, by gneiss and mica schist, and on the east 

 chiefly by granite. The vast volcanic eruption of the Plomb du Can- 

 tal, the highest point of which is 5571 French feet above the sea, 

 burst out within the area of this ancient and elevated lacustrine de- 

 posit long after the consolidation of its strata, which have in conse- 

 quence been fissured in every direction from that great centre, and 

 covered both by igneous and aqueous dejections ; the limestone and 

 marls being capped with sloping terraces of breccia and basalt, while the 

 streams flowing from the central heights have widened the fissures into 

 deep valleys. Two of the principal of these valleys, which radiate in a 

 westerly direction from the Plomb, are occupied by the rivers Cer and 

 Jourdanne, which unite near Aurillac, where the volcanic matter being 

 about twenty-five miles from its point of eruption, has thinned out to 

 a few irregular cappings, and consequently the lacustrine strata are 

 there least obscured. 



From an examination of numerous escarpments, the details of which 

 are given in separate sections, the authors establish the following de- 

 scending order. 



