408 THE JOURNAE OF GEOLOGY. 
appears to have issued from fissures near the summit of the 
range, mixed with water, forming enormous mud flows which cov- 
ered a large part of the western slope of the range. At Poker 
Flat (Downieville atlas sheet) Canyon Creek cuts across a huge 
fissure filled in part with rubble of the adjacent rocks, but chiefly 
with fragmental andesite. This was at first taken to be evidence 
that the mud flows became such before issuing from the inner 
regions, that is to say, the volcanic material was thought to have 
been broken and mixed with water within the earth. The creek 
has cut to a depth of 1000 feet or more through this fragmental 
dike, without reaching the bottom of it. During the past season, 
however, another similar occurrence was found, where it is plain 
that the fissure was filled from above. This second instance is 
about one and one-fourth miles south of Cammel Peak (Bidwell 
Bar atlas sheet), in the canyon of Fall River. The stream has 
cut into this second fragmental dike of andesite-breccia to the 
depth of about 500 feet, and in the dike material in the bed of 
the river are imbedded numerous fragments of fossil wood. 
These pieces of wood must have been washed into this fissure 
from the surface together with the andesitic material in which 
they are imbedded. The specimens of wood collected were 
referred to Professor F. H. Knowlton, who reports that ‘it is a 
Sequoia of the redwood, or S. sempervirens type. The wood is 
not well enough preserved to enable me to say that it is the same 
as the living redwood, although it is undoubtedly near it.” 
Fragmental volcanic rocks are usually supposed to have been 
formed by explosive action, the material being thrown out as 
fragments and ashes, which, falling on the surrounding land, or 
in bodies of water, would in the former case assume a roughly 
and in the latter a definitely stratified shape. But this does not 
seem to form an adequate picture of the Pliocene andesitic erup- 
tions of the Sierra. As before stated, the fragmental andesite 
appears to have been mixed with water, perhaps derived from 
melting snow at or very near the sources of the eruption. But 
however these mud flows were formed, on their course down the 
slopes of the range they caught up much foreign material. Silici- 
