192 
OLDER PLIOCENE PERIOD. 
[Ch. XIV. 
necessary to fill it up with earth and stones, in order to form a 
sure foundation for the new edifice. 
The annihilation of the ancient Olot may, perhaps, be as- 
cribed, not to the extraordinary violence of the movement on 
that spot, but to the cavernous nature of the subjacent rocks ; 
for Catalonia is beyond the line of those modern European 
earthquakes which destroy towns throughout extensive areas. 
As we have no historical records, then, to guide us in regard 
to the extinct volcanos, we must appeal to geological monu- 
ments. We have little doubt that some fossil land-shells, and 
bones of quadrupeds, will hereafter reward the industry of col- 
lectors. If such remains are found imbedded in volcanic ejec- 
tions, the period of the eruptions may be inferred ; but at 
present we have no evidence beyond that afforded by super- 
position, in regard to which the annexed diagram will present 
to the reader, in a synoptical form, the results obtained from 
numerous sections. 
No. 47. 
Superposition of rocks in the volcanic district of Catalonia. 
a, Sandstone and nummulitic limestone. c, Cones of scoriae and lava. 
b, Older alluvium with volcanic pebbles. d, Newer alluvium. 
The more modern alluvium d is partial, and has been formed 
by the action of rivers and floods upon the lava ; whereas the 
older gravel, b, was strewed over the country before the vol- 
canic eruptions. In neither have any organic remains been 
discovered, so that we can merely affirm, as yet, that the vol- 
canos broke out after the elevation of some of the newest rocks 
of the secondary series, and before the formation of an allu- 
vium, d, of unknown date. The integrity of the cones merely 
shows that the country has not been agitated by violent earth- 
