russell.] LAVA STREAMS. 83 
bank of the stream is of reddish tuff and is a part of the parent eone; 
the left bank is composed of large angular blocks of black lava, and is 
due to the subsidence of the central part of the stream, which left its 
border stranded. 
One of the most striking, if not the most novel of the many inter- 
esting features connected with the occurrence just mentioned, is the 
fact that large masses of tuff torn from the crater's walls were floated 
down the lava stream for at least 3 or 4 miles, and carried in large 
numbers far out on the adjacent plain. The floated masses are rough 
crags, frequently measuring from 20 to 60 feet in diameter and rising 
15 to 20 feet above the bare, black, corrugated surface in which they 
are partially embedded. About the base of each of these masses which 
was closely examined there is a depression in the surface of the lava, 
forming a moat-like trough 4 to 6 feet deep. It is plainly evident 
that the masses of loosely compacted lapilli and scoriae were of less 
specific gravity than the molten lava on which they fell, and therefore 
floated on its surface. How deep below the surface of the surround- 
ing lava the bases of the crags are sunken is not definitely known, but 
in one instance fissures formed since the lava hardened reveal a down- 
ward extension of the tuff to a depth of 10 or 12 feet. That these 
floated masses came from the tuff cone from which the lava stream 
emerged is plainly evident, not only on account of the material com- 
posing the masses themselves, which is the same as that of the cone, 
but for the reason that certain of the crags are near their place of 
origin, having baen carried but a few hundred yards, while others 
form a continuous train reaching faraway and expanding to the north- 
ward. Not only were masses of relatively light tuff floated away, but 
blocks of compact basalt which fell from the precipitous western bank 
after the central part of the lava stream subsided were carried several 
hundred feet on the flow during its later stage, when the molten rock 
had become exceedingly viscous. 
Another feature in this connection is of interest. The lava which 
flowed away from the Cinder Buttes was at first highly liquid, as will 
be shown more fully later, and spread widely on the adjacent plain, 
but as the supply lessened the lava thickened, becoming exceedingly 
viscous before it finally ceased to flow. The rate of flow toward the 
end of an eruption was characterized by a slowness as well marked as 
was the rapidity of the current during its earlier stages. The floating 
fragments on the lava, described above, fell upon it late in its liquid 
state, when it had become viscous and was partially submerged, press- 
ing down the stiff plastic material about them in the same way that a 
block of stone would indent the surface of a mass of highly viscous 
pitch or asphaltum. Whether the material that was carried away 
earlier was floated or sank is not known, as the broadly expanded 
distal extremity of the lava stream has not been examined. 
Although it frequently appears that lava streams in various regions 
