100 
ELEMENTS OF GEOLOGY. 
hope that we should ever be able to account for all the allu¬ 
vial phenomena of each particular country, seeing that the 
causes of their origin are so various. Besides, the last opera¬ 
tions of water have a tendency to disturb and confound to¬ 
gether all pre-existing alluviums. Hence we are always in 
danger of regarding as the work of a single era, and the 
effect of one cause, what has in reality been the result of a 
variety of distinct agents, during a long succession of geo¬ 
logical epochs. Much useful instruction may therefore be 
gained from the exploration of a country like Auvergne, 
where the superficial gravel of very different eras happens 
to have been preserved and kept separate by sheets of lava, 
which were poured out one after the other at periods when 
the denudation, and probably the upheaval, of rocks were in 
progress. That region had already acquired in some degree 
its present configuration before any volcanoes were in ac¬ 
tivity, and before any igneous matter was superimposed 
upon the granitic and fossiliferous formations. The pebbles 
therefore in the older gravels are exclusively constituted of 
granite and other aboriginal rocks; and afterwards, when 
volcanic vents burst forth into eruption, those earlier alluvi¬ 
ums were covered by streams of lava, which protected them 
from intermixture with gravel of subsequent date. In the 
course of ages, a new system of valleys was excavated, so 
that the rivers ran at lower levels than those at which the 
first alluviums and sheets of lava were formed. When, 
therefore, fresh eruptions gave rise to new lava, the melted 
matter was poured out over lower grounds; and the gravel 
of these plains differed from the first or upland alluvium, by 
containing in it rounded fragments of various volcanic rocks, 
and often fossil bones belonging to species of land animals 
different from those which had previously flourished in the 
same country and been buried in older gravels. 
The annexed drawing (Fig. 81) will explain the different 
heights at which beds of lava and gravel, each distinct from 
the other in composition and age, are observed, some on the 
flat tops of hills, 700 or 800 feet high, others on the slope of 
Fig. 81. 
Lavas of Auvergne resting on alluviums of diiferent ages. 
