302 Dr Daiibeny on the Ancient Volcanoes of Auvergne* 
disseminated crystals of hornblende, having frequently an aci- 
cular form, and sometimes a few of olivine. Associated with it 
is a vesicular rock, to which the name of Lava may not impro- 
perly be given, since the cavities which it contains possess no 
appearance of having once been Jilled^ like those in the amygda- 
loids of basaltic countries, and its general aspect obliges us to 
refer it to the same class with the undisputed products of vol- 
canoes now in activity. We meet also with scoriae, either in 
detached fragments, or with portions of compact trap, the whole 
cemented together by iron-clay, so as to constitute a species off 
volcanic tuff. I found the order of superposition on a hill 
which I examined near Lake Gery, a few miles from the village 
called Les Bains de Mount dDr,” to be as follows : 
On the summit a thin bed of scoriae. Underneath, a tuffj 
containing fragments of the more compact, united with the vesi- 
cular variety of lava, but in some places in a state of such ex- 
treme division, that the whole might be mistaken at a distance 
for red sandstone. Beneath all was a very compact and crys- 
talline basalt, made up of a confused assemblage of these acicu- 
lar crystals of hornblende, crossing each other in all directions, 
intermixed with felspar. 
It would seem that this is the general order of superpo- 
sition ; and the pressure of the vesicular matter above may pos- 
sibly have contributed to give to the rock underneath the hard- 
ness and consistency which it is found to possess. 
Thus, we may observe in many of the recent volcanic pro- 
ducts near Clermont, that the masses which lie underneath are 
less vesicular than those near the surface, although I have noti- 
ced in my former communication one fact which seems to mili- 
tate against the universality of this remark. It is also true, 
that I have found basalt among the mountains of Auvergne, un- 
covered by any more porous rock ; but is it not probable, that 
the same causes which excavated the valleys, and removed all 
traces of the craters, which we must suppose to have once ex- 
isted, should have in these instances swept away the upper strau 
ta, which, from their looser and more vesicular structure, were 
less able to oppose resistance ? May not this account for the oc- 
currence of clinkstone, the least decomposable of trap-rocks, 
chiefly on the summit of hills ; for this position seems hardly 
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