48 J. S. Diller—Latest Volcanic Eruption im - 
Lake. Before reaching the surface the basalt of the necks 
traversed several thousand feet of horizontal shales and sand- 
stones which in some places are so friable as to easily crumble 
in the hand. A more favorable opportunity than this cannot 
be imagined for the basalt at the time of its extrusion to be- 
come thoroughly impregnated with grains of sand. Neverthe- 
less no trace of quartz grains or small fragments of sandstone 
is found a short distance inside of the boundary of the neck. 
The abundance of the quartz grains in the large volcanic 
bombs is a fact of special significance. It is evident that at 
the time of the eruption the bombs were thrown up into the 
air and falling upon the steep slopes of the Cinder Cone they 
tumbled to its base, where they collected in great numbers. 
The bombs vary in size from half a dozen inches to over eight 
feet in diameter. ‘They are nearly round, and toward the 
center are very compact and jointed frequently in such a man- 
ner that the fissures radiate from the center of the bomb. The 
surface is rough, often vesicular and pumiceous. If the bombs 
had been soft at the time of their ejection as is frequently the 
case, they would have flattened out when they struck the 
eround, but from the fact that they remained round it may be 
inferred that they were already solid at the time of the erup- 
tion, having previously floated around as clots in the magma. 
These bombs are undoubtedly the oldest portion of the lava, 
having solidified previous to the eruption or in its earliest 
stage, before the magma in which they were suspended reached 
the surface. The fact that the grains and lumps of quartz are 
abundant to the very center of the largest bombs demonstrates 
that they are early secretions from the magma at a time when 
it was under enormous pressure. During the eruption the 
pressure was relieved and the consequent conditions of solu- 
bility changed, so that the quartz secretions, like those of the 
more acid lavas, were partially redissolved or fused,.giving rise 
to the zone of glass and pyroxene by which the quartz is sur- 
rounded. 
The quartz basalt is younger than any of the many basalts 
about it. There are numerous cinder cones and recent flows 
of basalt upon all sides of the one under consideration, and 
yet, although they were extravasated through the same forma: - 
tions and under the same conditions as the quartz basalt, they 
contain no quartz. This fact indicates clearly that the source 
of the quartz is to be found in that of the basalt itself. 
Until recently I had supposed that there was only one local- 
ity of quartz basalt in that region, but later investigations in 
the field have discovered another near Silver Lake, twenty-five 
miles northwest of Lassen Peak, about an entirely distinct vol- 
eanic center. It has all the peculiar features of the quartz 
