russeli.] RECENT VOLCANOES. 53 
streams down moderate^ steep slopes and made for itself well-defined 
channels. These channels are, in general, abont 3 feet wide, 3 to 4 
feet deep, with the crests of their bordering ridges elevated about 3 
feet above the adjacent surface, and from 50 to 150 feet long. One 
example of these troughs or gutters, situated at the south base of the 
driblet cone shown on PI. XI, A, is about 50 feet long, 14 to 16 
inches wide, and 3£ to 4i feet deep; the crests of the bordering ridges 
are, in general, 3 feet above the adjacent surface. Near its distal 
extremity the trough is complete^ roofed over, as may be seen in the 
illustration just referred to. The largest example observed is situ- 
ated on the east side of Crater No. 4, and is about 150 feet long, 3 feet 
wide, with well-defined walls, which rise approximately 3 feet above 
the neighboring surface. These and other similar gutters usually 
begin abruptly at their upper ends and retain a nearly uniform width 
and depth to near their distal extremities, where their walls become 
lower and the troughs merge with broadly expanded, low, dome-shaped 
elevations on a general pahoehoe surface. In some instances the 
liquid lava, as it flowed through one of these gutters, became cooled 
at the surface so as to form a crust, which was left as a slightly 
arched roof above a tunnel as the still-liquid lava beneath flowed out. 
These gutters with roof are in fact tubes or tunnels. In the instance 
shown in PL XIV, a well-defined gutter was formed, in which the 
lava flow decreased until the surface of the current was about 3 feet 
below the crest lines of the inclosing ridges; a crust was then formed, 
and the lava below it flowed out, leaving a roof spanning the gutter 
throughout its length. 
The mode of formation of lava gutters seems to be that a narrow 
stream of liquid lava cools at its margins and forms a slight ridge, 
more lava spreading laterally over this ridge, in turn cools and stiffens, 
and then adds to its height, and so the process continues until a well- 
defined gutter with raised borders is formed. These parallel ridges 
on the borders of a narrow, high-grade lava torrent are in a measure 
analogous to the natural levees built by alluvial-depositing streams. 
Lava flow. — The lava from Crater No. 4 of the Jordan series, as 
already stated, flows southeastward over a previous^ stream-eroded 
surface. The lava, just after leaving the crater from which it came, 
formed a stream 545 yards wide, and increasing rapidly in width 
reached a distance of about 8 or 10 miles. The entire flow is by esti- 
mate between 50 and 60 square miles in area. The average depth 
may perhaps be taken as approximately 100 feet. These statements, 
it must be remembered, are rough estimates, as no surveys have been 
made, and no maps of the region are available. The lava as it ad- 
vanced was guided in a conspicuous way by the preexisting topog- 
raph}', and in several instances progressed short distances up lateral 
valleys tributary to the main depression down which it flowed. One of 
these small offshoots went nearly due north for one-half mile up a small 
