46 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bull. 217. 
with a depression or crater at the summit. The outer slopes of these 
cones frequently form angles of 30° to 35° with a horizontal plain, 
the angle being essentially the angle of repose of the fragments 
of which they are built, but their profiles are concave instead of 
straight lines. The reason for the downward curves presented by 
the sides of such cones is not definitely understood, but seems 
to depend on the fact that the structures are built of fragments 
of different sizes and shapes. Vertical sections through such piles 
commonly sho.w, as is well known, two series of beds, one series 
dipping away from the crater in the summit, and forming the outer 
slopes of the cones, and the other series dipping from the rim of the 
crater toward its bottom. The junction between these two series of 
opposite-dipping beds in the rim of a crater is not a sharp angle, as 
sometimes represented, but a curve, convex upward. The cones 
referred to are symmetrical when built of fragments projected verti- 
cally, unless the influence of the wind in carrying the material in one 
direction more than another made itself felt. Cones or craters built 
of angular fragments with surfaces produced by fractures are suf- 
ficiently distinct from the similar structures built of other kinds 
of projectiles to be specially designated, and may with propriety be 
termed lapilli craters, or lapilli cones in case a depression in the sum- 
mit of the pile is absent. 
When the material projected into the air from a volcanic vent is 
plastic, and falls in irregular clots and rough scoriaceous masses, it 
frequently forms steep-sided piles with chimney-like openings within. 
Owing to the frequently large size of the masses which fall, their 
rough surfaces, and also to the fact that they are in many instances 
still plastic when they come to rest, the slopes of the structures pro- 
duced, both on the outside and within, are commonly steep, and in 
some instances are nearly vertical, but the outer slopes of these hollow 
piles are normally less steep than the walls of the openings within. 
Piles of congealed clots of this nature present several instructive 
variations. At times they are steep-sided chimney-like forms, such 
as are illustrated on PI. III. In other instances their encircling walls 
are contracted at the top so as to leave only small openings, and in 
extreme examples of this nature a complete roof is formed by the 
adhering of the clots blown out, and a beehive or oven-like structure 
results, such as is shown on PI. XL The chimney-like elevations pro- 
duced in the manner just referred to have been designated driblet 
cones by J. D. Dana, but for the entire series of cones, craters, etc., 
built of scoriaceous or cinder-like clots of lava the term cinder crater 
or cinder cone seems appropriate. 
For the sake of avoiding a lengthy discussion of the classification 
of elevations produced by the accumulation of volcanic projectiles 
about the vents from which they were blown out, I may briefly state 
that in certain instances well-characterized volcanic bombs form the 
greater portion of an elevation with a crater in its top, and in some 
