Dr Daubenj on the Ancient Volcanoes (^'Auvergne. . 315 
were^ to observe a series of experiments which Nature herself 
has instituted respecting the action of heat on solid bodies un- 
der different circumstancesj the results of which, it is presumed, 
are more likely to be conclusive than any which can be conduct- 
ed in our laboratories, allowing every credit to the talent of the 
individuals who conducted them ; and even admitting their im- 
portance with reference to tlie point in question, as corroborato- 
ry of what we may have observed on the great scale in coun- 
tries like Auvergne. I am far, indeed, from having the pre- 
sumption to suppose, that the facts recorded in this and the for- 
mer letter, are sufficient to decide the question ; but my purpose 
will be answered, should it contribute to excite some more accurate 
and acute observer to go over the same ground that I have 
done, convinced, that if ever the origin of the trap-rocks in our 
own and other countries be fully cleared up, it will be owing in 
a great measure to a due examination of the analogous forma- 
tions in Mont d’Or and in Cantab 
Before, however, I bring this letter to a close, there is one ob- 
servation which may be hazarded, without entering too much 
into theory, inasmuch as it seems immediately deducible, either 
from the foregoing facts, or from others which, I apprehend, will 
be pretty generally admitted. As there appears an almost un- 
interrupted transition from lava to basalt, from basalt into green- 
stone and sienite, from sienite into granite, and from granite in- 
to sandstone ; and since it is evident, that the two extremes in 
this apparently connected series must have originated from op- 
posite causes, we are brought at length to admit, that similarity 
of character in rocks is not sufficient to establish identity of ori- 
gin ; and that the structure, position and connexions of a rock 
on the great scale with those contiguous, must be taken into the 
account in each particular case, before we decide, whether the 
formation in question ought to be ranked among the products 
of Fire or Water. There seems at least to be the same resem- 
blance between certain varieties of sienite and greenstone, as any 
which can subsist between the trap-rocks in the north of Ire- 
land, and those which occur inter stratified with sandstone and 
calcareous beds onJ:he coast of Fife, near Edinburgh. Yet, how 
many geologists scruple to admit the igneous origin of thej^r- 
mevy from considering its connexion with granite, who, never- 
