36 Messrs Lyell and Murchison on the 



troughs ; and a variety of the same grit, at Mont Peyroux, not 

 far distant, is employed for mill-stones. Near Nechers, in the 

 same neighbourhood, it passes into a red and green sandstone, 

 is of great thickness, and cannot be distinguished in character 

 from the new red sandstone of many parts of England ; al- 

 though an examination of the tertiary strata of Auvergne has 

 satisfied us that it really constitutes one of the inferior members 

 of those great lacustrine deposites. 



As the raineralogical structui'e of this rock has, under the 

 denomination of Arkose, been minutely given by M. Brongniart, 

 when describing it in other parts of this district, we may refer 

 the reader to his memoir.* 



This grit is here covered by alluvium, which extends along a 

 plateau, stretching upwards from Les Chapelles to the fai-m of 

 Boulade. From its component parts, we may conclude this al- 

 luvion to have been derived from one of more ancient date, 

 hereafter to be described, and which has been partially de- 

 stroyed. 



In ascending the ravine near the farm of Boulade, the fol- 

 lowing beds are exposed in succession, beginning with the 

 lowestj-f- : — 



1*^, Thinly laminated, white and green marls abounding in 

 cypris, with some bands of siliceous limestone, about twelve 

 feet thick. These strata dip at an angle of ten degrees WSW. 

 and evidently form part of the tertiary lacustrine formation, so 

 widely developed in this part of Auvergne. 



ScZ, Unconformably upon these rest beds of fine and coarse 

 sand and gravel, alternating with each other to the thickness of 

 eighteen feet. The pebbles are chiefly of basalt and quartz, 

 the cement sandy, and sometimes ferruginous. 



Sd, These are surmounted by a bed of sandy gravel, of an 

 ochreous colour, varying in thickness from nine inches to two 

 feet, whence the greater part of the fossil bones which have 

 given so much celebrity to this hill have been extracted. Above 

 this sand is a bed of black and ferruginous gravel, two feet in 

 thickness. 



• Sciences Naturelles, June 26, sur 1' Arkose. f See Plate III. 



4 



