68 Dr Daubeny on the Colimnar Structure of Trap-Rochs. 
appear to be bent in conformity to the slope of the hill. The 
curve which they describe still increasing, they become by de- 
grees quite horizontal, so that the terminal planes only of the 
prisms jut out from the side of the rock. Now, it is curious, 
that, in the latter instance, the interstices between the columns 
4ire generally filled up by a species of trap much looser in its 
texture than that of which the columns are themselves compos- 
ed, the want of consistency in the stone appearing to increase in 
proportion as it recedes from the prismatic portion of the rock. 
In this case, therefore, we seem to have an instance of a rock, 
the distinct concretions of which are obscured by an upfilling 
(if I may be allowed the expression) of matter possessing in- 
ferior hardness and looser consistency * that in such cases the 
abrading action of water would be competent to effect the re- 
moval of these softer portions, and the consequent exposure of 
the structure underneath it, will, I think^ appear from the fol- 
lowing circumstance, observed in the course of my travels 
through the same part of France, which furnished me with the 
fact I have just been mentioning. 
At the village of Prentigarde, about half a league from “ Les 
Bains de Mont Dor,^’ a well known watering-place in the de- 
partment of the Puy de Dome, the trachytic or porphyry for- 
mation, of which I have given a detailed account in my Memoir 
OB Auvergne, is seen surmounted by a basalt of a very compact 
character. A good section of the two rocks is exhibited at 
what is called the Cascade du Quereuil, where a small moun- 
tain torrent is precipitated down a perpendicular escarpment, to 
the depth of perhaps 50 feet , 
Now, it is curious, that although the basalt on either side is 
w'holly amorphous, being divided only by irregular fissures in- 
to imperfect quadrangular masses; yet that, where it lies in 
• 1 did not observe any of this firiable trap filling up the angles between those 
columns which approached to a vertical position. Under these circumstances, 
they might have been freely exposed to the denuding action of water at some for- 
mer period. The columnar structure is therefore here fully developed ; but where 
the columns have, by some singular convulsion of nature, or the operation of some 
unknown cause, been rendered horizontal, the amorphous portions filling up the 
interstices between the columns, were in some measure protected from the aation 
.of the elements, and there accordingly they remain. 
