VARIOUS VOLCANIC PHENOMENA. 265 
We traced one of these volcanic chambers to 
the edge of the precipice that bounds the great 
crater, and looked over the fearful steep down 
which the fiery cascade had rushed. In the place 
where it had fallen, the lava had formed a spa¬ 
cious basin, which, hardening as it cooled, had 
retained all those forms which a torrent of lava, 
falling several hundred feet, might be expected to 
produce on the viscid mass below. In the neigh¬ 
bourhood we saw several large masses of basaltic 
rock, of a dark gray colour, weighing probably 
from one to four or five tons, which although they 
did not bear any marks of recent fire, must have 
been ejected from the great crater during some 
violent eruption, as the surrounding rocks in 
every direction presented a very different appear¬ 
ance ; or they might have been thrown out in a 
liquid state, combined with other matter that had 
formed a rock of a less durable kind, which, de¬ 
composing more rapidly, had been washed away, 
and left them in detached masses on the plain. 
They were hard, and, when fractured, appeared a 
lava of basalt, containing very fine grains of com¬ 
pact felspar and augite; some of them contained 
small particles of olivine. We also saw a number 
of other rocks in a state of decomposition, which 
proved to be a species of lava, containing globules 
of zeolite. The decomposition of these rocks ap¬ 
peared to have formed the present surface of much 
of the west, north, and east parts of the plain im¬ 
mediately surrounding the crater. 
When we had broken off specimens of these, 
and of some red earthy-looking stones, which 
seemed to have the same base as the other, but 
to have lost their compact texture, and to have 
experienced a change of colour from a farther 
