russell.J SURFACE FEATURES AROUND CINDER BUTTES. 89 
liquid, but onty viscous, and indeed in some observed instances nearly 
solid. The quicker setting of siliceous than of basic slag's suggests 
also a reason why the surfaces of acid lavas are so frequently broken 
and consist of masses of angular fragments, without the smooth, oval, 
and flowing forms that basic lava frequently presents. 
SURFACE FEATURES. 
All of the recent lava streams about the Cinder Buttes, as previously 
stated, are similar in their general characteristics, but when examined 
in detail they present interesting variations in surface features, not 
only in individual streams, but in different portions of the same 
stream. 
In general, the lava at a distance of from 5 to 10 or more miles from 
its place of emergence is less vesicular than at the summits of the 
conduits from which it came. The reason for this is, evidently, that 
the steam occluded in the molten rock had greater facility to escape 
when the lava flowed far than when its journey was short. More 
thai] this, the lava first extruded from a vent and now found at the 
distal extremity of the stream it formed flowed much more rapidly 
than that which came later, either because of a higher temperature 
and consequently greater liquidity, or by reason of larger volume. 
To these two alternatives a third may be postulated, namely, a varia- 
tion in the composition of the lava, which caused it to become viscous 
near the close of the discharge without a diminution of temperature. 
While analyses of the lava that flowed far have not been made, the 
physical appearance of the distal and proximal extremities of the 
streams do not suggest a difference in composition, and this third 
alternative can seemingly be disregarded. Between the remaining 
hypotheses there is an easy choice, namely, that the rate of flow of 
the lava diminished without a decrease in the initial temperature. 
This is evident, since if the rate of flow had remained essentially the 
same from the beginning to the close of a discharge, and the lava had 
become viscous toward the end of an eruption, it would have acquired 
a corresponding increase in depth near its source, and if as viscous as 
the lava now remaining at the summits of the conduits was before 
hardening, it would have been piled high above the opening from 
which it was extruded. On the contrary, we find but a slight increase 
in surface gradient on ascending any one of the lava streams near its 
source. All the evidence indicates that the volume of the lava toward 
the close of an eruption gradually diminished until the outflow ceased. 
The last lava to reach the surface cooled and hardened without lateral 
motion. 
These considerations have a bearing on the theories of the causes of 
volcanic eruptions. Of these there are two that have claimed special 
attention. One is, in brief, that steam is the main motive power 
