rtjssell.] HOLES IN LAVA SHEETS. 101 
it is remembered that the vertical shafts are principally in the built- 
up portions of the "chimneys," and hence the material blown out 
must have been supplied in large part by the horizontal galleries to 
which the vertical shafts lead. 
In some instances parasitic or "driblet" cones, are closed at the top 
so as to form beehive-like piles. These peculiar structures, built of 
adhering clots of lava, sometimes contain circular rooms that are 15 to 
20 feet in diameter and 15 or more feet high. Their floors are usually 
flat and their arched sides and ceilings are hung with pendent masses 
of lava which cooled as it dripped. 
DEPRESSIONS IN THE SURFACES OP LAVA SHEETS. 
The falling in of cavern roofs so as to produce open depressions in 
the surface of a lava stream has already been referred to. This has 
been a common occurrence on the younger flows, such as those about 
the Cinder Buttes, Black Butte, etc. These holes, pits, trenches, etc., 
are frequently 15 to 20 feet in diameter and 10 to 20 or perhaps 50 
feet deep, but ragged gulfs of much greater dimensions are also pres- 
ent, especially on Black Butte. In some instances they are several 
times as long as wide, or have a linear arrangement showing that the 
roof of a long subterranean gallery has fallen in throughout a consid- 
erable part or perhaps the whole of its length, or else given way in 
places, producing several pits. When the fall of a cavern roof has 
occurred recently its fragments may be recognized in the confused 
pile of debris at the bottom of the depression left, but more f requentty 
quantities of dust or soil have been blown and washed in, and bushes, 
taking root, conceal the dislodged fragments. 
In a few instances the roofs of caverns have been known to fall, 
and the liability of such an occurrence makes traveling over the 
younger lava fields somewhat dangerous. I have been informed that 
only a few years ago, in the portion of the lava flow south of Cinder 
Buttes, a team of horses and a wagon broke through a cavern roof and 
went down with the blocks that fell. 
On the surfaces of the older lava flows, now mostly covered with a 
fine yellowish soil, pits and steep-sided depressions of the nature just 
described are rare and seemingly wanting. The reason for this is, 
apparently, that they have been tilled with wind-borne debris. On the 
surfaces of the older flows there are, however, occasional shallow depres- 
sions from a fraction of an acre to several acres in extent which are 
bordered in part or wholly by ridges of lava. These depressions are, 
in many instances, occupied by water during the rainy season and 
transformed into shallow, ephemeral pools, of the variety termed 
playa lakes. In summer the water evaporates, leaving mud flats or 
playas. The depth of the fine silt-like material occupying these depres- 
sions is known from borings, in one instance, to be about -10 feet. It 
