russell.1 TERTIARY FORMATIONS. 59 
tures formed of material which was ejected in various conditions 
ranging from extreme viscosity to liquidity, and which formed scoria, 
bombs, lava cakes, and splashes of lava which became confluent 
after falling. The elevations built by the accumulation of such pro- 
jectiles range, as has been stated, from lapilli cones or craters, due 
to the piling up of solid angular fragments, through similar craters 
composed in part or perhaps wholly of clots of plastic lava, volcanic 
bombs, and lava cakes to driblet cones and "ovens." The range in 
angle of slope on the exterior of these structures is from the neigh- 
borhood of 30° for the loose angular fragments to nearly vertical for 
the adhering clots; the range in slopes of the interior of the crater 
walls is about the same as for the exterior. When the driblet cones 
are roofed over, their inner slopes, both on the outside and within, 
pass beyond the vertical and approach the horizontal. 
The lava flows also present a wide range in the resulting details, 
such as typical pahoehoe (PI. XII, A) with corrugated surfaces, and 
even hollow folds due to the influence of an underflow beneath a 
plastic crust, and equally typical aa (PI. XII, B), due to the breaking 
of a rigid crust on account of the energetic flow of still liquid lava 
beneath it. The occurrence of lava gutters (PL XIV) and the pres- 
ence, in numerous instances, of prominent pressure ridges on the 
recent lava sheets (PI. XV, B) are also instructive features. The thin 
margins of liquid lava streams and the conspicously abrupt and rugged 
extremities of highly viscous lava flows are illustrated on PI. XIII. 
All the material extruded from the volcanoes described above is 
basalt, and represents the more easily fusible of lavas. 
In many ways these modern volcanoes serve to illustrate the nature 
of the far larger outporings of molten rock which, on cooling, formed 
the Snake River Plains and the still vaster, but, in part at least, 
somewhat differently erupted Columbia River lava. In a general 
discussion of volcanoes the Cinder Buttes, etc. , may be considered as 
representing volcanoes of the quiet type, or such as erupt without 
energetic explosions, but their periods of activity were in all cases, so 
far as can be judged, initiated by explosions sufficiently violent to 
blow out projectiles, which on falling formed conspicuous elevations. 
They may be considered, therefore, as furnishing a connecting link 
between volcanoes of the explosive and those of the quiet type. These 
intermediate examples indicate, as has long been recognized, that the 
classification of volcanoes under two types, viz, explosive and quiet, 
is largely for convenience and that no sharply defined boundary sep- 
arates the two. 
TERTIARY FORMATIONS. 
Besides the products of recent volcanoes, briefly described in the 
preceding pages, there are, in the portions of Idaho and Oregon under 
consideration, widely extended sheets of rock which may be classified 
according to their mode of origin in two broad groups of terranes, one 
of which includes the rocks composed of material that was deposited 
