212 On the Antiquity of the Volcanos of Awvergne. [April, 
Now, when the bottom of the valley is occupied by running 
water, its bank is walled in by a colonnade of basalt, extending for 
a considerable distance along its margin, derived from the lava 
stream which had descended from the mountain above. 
It is true that when, as sometimes happens, the igneous mass 
is not perceived on the opposite bank of the river, as is the case 
at the Coupe de Col, d’Aisac, of which a description and drawing 
has been given by Faujas St. Fond, we have no right, for the 
reasons above stated, to ascribe the entire height of the vertical cliff 
of basalt to the eroding force of water; but, in other cases, as at 
the spot called the Gravenaire of Montpeset, of which Mr. Scrope 
has given us a drawing, there can be no mistake about the matter, 
as a high, precipitous rock, upon which the ruins of a castle stand, 
is severed from the main body of the lava current, and rises up in 
the midst of the stream. ‘The upper portion of this rock is 
composed of basaltic lava, derived from the mountain above 
and forming the termination of a current which had flowed 
from it; but the lower consists of gneiss, which, since the lava 
current had been erupted, is seen to have been excavated by the 
erosive power of the stream to the depth of 100 feet. The time 
necessary to bring about this effect I will not pretend to estimate, 
but may appeal to it as a proof of the great antiquity of a lava 
current, which must have, at least, been antecedent to its com- 
mencement. 
One very remarkable peculiarity of the lava streams in the 
Vivarais currents is their basaltic character and their prismatic 
structure. We are accustomed to consider trap rocks in general, 
and more especially that particular description which is denominated 
basalt, as exclusively the product of submarine volcanos, their com- 
pactness being said to arise from the great pressure exercised upon 
them during their consolidation. But in this part of France we 
meet with several instances of basaltic colonnades, which have been 
evidently derived from streams of lava ejected from sub-aerial 
volcanos. 
It is true that, in all those specimens which have come under 
my notice, minute cells and cavities may be discovered by careful 
examination, and, moreover, that the upper portions of the bed are 
more pervaded by them than the lower. 
Still the resemblance which they bear to the products of 
submarine volcanos is very remarkable, and only admits of being 
explained by the thickness of the bed and the weight of the scoriz 
superimposed, for it evidently matters not in what way the pressure 
is produced, provided it be sufficient to retain the aqueous and 
other volatilizable. ingredients present within the rock in such a 
condition as to prevent the production of cells and cavities. 
And, accordingly, it is observed, that this compact character and 
ae arti 
