russell.] RECENT VOLCANOES. " 37 
known to geologists, also examined. The facts observed in connec- 
tion with these four modern and still well-preserved volcanoes or 
groups of volcanoes, when taken in connection with the studies already 
made of many other similar examples in the same region, not only 
contribute to a better understanding of volcanic phenomena in gen- 
eral, but aid in an important way in the interpretation of the records 
of the far more extensive lava flows of greater age in the same region. 
The more instructive features of the recent volcanoes of Idaho, 
described in the report referred to above, may be briefly summarized 
as follows : The lava extruded is in all instances a dark basic rock, 
and may all be classed as basalt. In physical characteristics it ranges 
from compact, columnar rock to highly scoriaceous and even pumice- 
like material. The lava was extruded from the volcanic conduits in 
two ways: part, and by far the larger part, flowed out in a highly 
liquid condition and spread widely over the surface before cooling 
and hardening, and part was blown into the air as angular fragments 
or as clots, bombs, etc., which fell on the adjacent surface and built 
more or less conspicuous elevations. The material extruded in each 
of these ways presents many variations in its physical condition, 
dependent on secondary influences. 
Of the lava which flowed out in streams two principal varieties 
may be recognized ; one division includes the rocks formed by the 
cooling of the molten material on land, which, although at times highly 
scoriaceous, especially at the surface of the sheets, is in general a 
dense, compact rock, and on account of the formation of joints as it 
cooled is frequently columnar; the other division contains the lavas 
which entered surface water bodies and became expanded and 
" shredded," or shattered by the resulting steam explosions. The lava 
which was blown into the air presents an interesting series of forms, 
ranging from angular fragments produced by the shattering of the 
material after it had become cool and rigid, through clots, bombs, 
lava cakes, etc. , formed of more and more plastic lava, to splashes of 
the magma which were still highly liquid when they fell. 
From the two principal variations in the manner in which the vol- 
canoes discharged their lava, two types of elevations about the mouths 
of their conduits resulted. The explosive eruptions led to the build- 
ing of lapilli and cinder cones, a considerable range in the forms pro- 
duced resulting from the degree of rigidity or plasticity of the pro- 
jected fragments and the extent to which they cooled before striking 
the surface. These elevations are usually steep sided and contain 
depressions or craters in their summits, and as a class may be termed 
cinder cones. The lava overflows occurred for the most part quietly, 
without explosions, although in general, as it is believed, following 
explosive eruptions which opened the volcanic conduits for the free 
discharge of the molten rock which rose later and was poured out in 
such volume that the extruded material cooled in a mound above the 
