Excavation of Valleys. 33 



right liand side. The granite beneath this alluvion is worn into 

 to the depth of about 100 feet. We also remarked, near Besse, 

 in the valley o^ the Couze of Issoire, the same river which in 

 a lower part of its course runs at the foot of Mount Perrier, a 

 still more striking example of two lava currents, of distinct ages, 

 which had both flowed in the same direction. The most recent 

 has all the characters of a modern "cheire," and may be seen a 

 short distance above the village of Ourseyre, which is built 

 upon it, to be worn into by the river about 50 feet. Above it, 

 and on the right bank of the Couze, an older current of colum- 

 nar phonolite is seen^ die base of which descends very nearly as 

 low as the most elevated points of the uneven " cheire." But 

 a talus of fallen prisms preveq^ed our ascertaining whether the 

 phonolite rested on an ancient alluvion. Although in such 

 marked cases we cannot refuse to admit the higher and older of 

 the two currents to be of an intermediate age, in reference to 

 the volcanic products of Auvergne ; and although we grant that 

 the terms " ancient " or " modern " may both be equally misap- 

 plied in regard to such rocks, we are not prepared to consider 

 the same test of relative elevation as indicating the age of an- 

 cient plateaus of basalt, forming the cappings of insulated hills, 

 or lofty ridges, and whose position has no visible relation to 

 existino- valleys. In a country where the granite has subse- 

 quently given vent to so many separate volcanic eruptions, and 

 where there are proofs, as might have been expected, of its 

 havino- undergone considerable subsequent convulsions ; where 

 the tertiary strata, from the uppermost to the lowest member in 

 the series, are found to dip in almost every direction, and at 

 every inclination from vertical to horizontal, — and where they 

 are traversed by fissures, faults, and basaltic dikes (the latter 

 not only causing disturbance, but continuous, in some instances, 

 with overlying plateaus) ; — to attempt, in such a district, to es- 

 tablish relative elevation as a criterion of age, seems almost as 

 unsafe as to determine the respective antiquity of overlying 

 trap-rocks in Scotland by the same standard. 



Some travellers have considered the ancient plateaus of basalt 

 in Auvergne as implying a discrepancy between the manner of 

 the ancient and modern volcanic action, for there was a greater 



APRIL JUNE 1829. ^ 



