CHAPTER XIV. 
Volcanic rocks of the older Pliocene period — Italy — Volcanic region of Olot in 
Catalonia — Its extent and geological structure — Map — Number of cones — ■ 
Scoriae — Lava currents — Ravines in the latter cut hy water — Ancient alluvium 
underlying lava — Jets of air called ' Bufadors ' — Age of the Catalonian vol- 
canos uncertain — Earthquake which destroyed Olot in 1421 — Sardinian vol- 
canos — -District of the Eifel and Lower Rhine — Map — Geological structure 
of the country — Peculiar characteristics of the Eifel volcanos — Lake craters — 
Trass — Crater of the Roderberg — Age of the Eifel volcanic rocks uncertain — ■ 
Brown coal formation. 
VOLCANIC ROCKS OF THE OLDER PLIOCENE PERIOD. 
Italy . — It is part of our proposed plan to consider the igneous 
as well as the aqueous formations of each period, but we are 
far from being able as yet to assign to each of the numerous 
groups of volcanic origin scattered over Europe a precise place 
in the chronological series. We have already stated that the vol- 
canic rocks of Tuscany belong, in great part at least, to the older 
Pliocene period, — those for example of Radicofani, Viterbo, 
and Aquapendente, Avhich have been chiefly erupted beneath 
the sea. The same observation would probably hold true in 
regard to the igneous rocks of the Campagna di Roma. 
But several other districts, of which the dates are still 
uncertain, may be mentioned in this chapter as being possibly 
refer rible to the period now under consideration. It will at least 
be useful to explain to the student the points which require 
elucidation before the exact age of the groups about to be 
described can be accurately determined. 
Volcanos of Olot, in Catalonia. — I shall first direct the 
reader's attention to a district of extinct volcanos in the north 
of Spain, which is little known, and which I visited in the 
summer of 1830. 
The whole extent of country occupied by volcanic products 
in Catalonia is not more than fifteen geographical miles from 
