Ch. XIV.] RAVINES IN LAVA EXCAVATED BY RIVER?. 
189 
pebbles and angular fragments of rock ; in other places fine 
earth, which may have constituted an ancient vegetable soil. 
In several localities, beds of sand and ashes are interposed 
between the lava and subjacent stratified rock, as may be seen 
if we follow the course of the lava-current which descends from 
Las Planas towards Amer, and stops two miles short of that 
town. The river there has often cut through the lava, and 
through eighteen feet of underlying limestone. Occasionally 
an alluvium, several feet thick, is interposed between the igne- 
ous and marine formation ; and it is interesting to remark, that 
in this, as in other beds of pebbles occupying a similar position, 
there are no rounded fragments of lava, whereas, in the modern 
gravel beds of rivers in this country, volcanic pebbles are 
abundant. 
The deepest excavation made by a river through lava, which 
I observed in this part of Spain, is that seen in the bottom of 
a valley near San Feliu de Pallerdls, opposite the Castell de 
Stolles. The lava there has filled up the bottom of a valley, 
and a narrow ravine has been cut through it to the depth of 
100 feet. In the lower part the lava has a columnar structure. 
A great number of ages were probably required for the erosion 
of so deep a ravine ; but we have no reason to infer that this 
current is of higher antiquity than those of the plain near Olot. 
The fall of the ground, and consequent velocity of the stream, 
being in this case greater, a more considerable volume of rock 
may have been removed in an equal quantity of time. 
We shall describe one more section to elucidate the phe- 
nomena of this district. A lava-stream, flowing from a ridge 
of hills to the east of Olot, descends a considerable slope 
until it reaches the valley of the river Fluvia. Here, for the 
first time, it comes in contact with running water, which has 
removed a portion, and laid open its internal structure in a 
precipice about 130 feet in height, at the edge of which stands 
the town of Castell Follit. 
By the junction of the rivers Fluvia and Teronel the mass 
of lava has been cut away on two sides ; and the insular mass 
