402 BOTANICAL GAZETTE [NOVEMBER 
from the many crevices or cavities. Whatever showers fall on this portion of 
Hawaii are at once absorbed by the cavernous rocks; and consequently through 
its whole extent, south and east, there are not two permanent streamlets. 
Water is to be found only in caverns; and often a journey of some miles must 
be taken by the villager to supply himself for his daily consumption. All the 
caverns about the lower parts of the mountains have been well explored for 
this necessary of life. 
There is probably no other region in the world where rainwater 
disappears with greater rapidity than on the leeward slopes of 
the Hawaiian Mountains. The honeycombed lava flows swallow 
it up, and convey it to the sea through deep subterranean channels. 
Thus the aridity of the lava country is compounded by 3 factors: 
low precipitation, high evaporation, rapid percolation. 
EvAPORATION.—An_ ecological factor of probably greater 
importance than either precipitation or percolation is that of high 
evaporation, which characterizes the Hawaiian flows, as it does 
all arid regions. This very high evaporation is strongly productive 
of xerophilous structures and is probably more potent than any 
other single factor. 
The lava flow 
In order to elucidate the structural peculiarities of the lava 
country as related to plant life, a brief synopsis of the formation of 
a typical flow may be presented. This is adapted from Hosss’s’ 
account. j 
The lava either quietly melts its way to the surface at the 
time of outflow, or else produces one or more fissures for its egress 
to the accompaniment of vigorous local earthquakes. In either 
case, if the lava issues at a point far below the crater, the hydro- 
static pressure causes gigantic lava fountains to arise at the point 
of outflow. The fluid, incandescent rock shoots up to heights 
which range from 200 to 700 ft: or more above the surface. In 
the 1852 eruption of Loa a fountain of lava 1000 ft. broad rose to 
a height of 700 ft. <A certain proportion of this fluid lava is suffi- 
ciently cooled to consolidate while traveling in the air, and upon 
falling it builds up a cinder cone. This cone becomes a location 
7 Hosss, W. H., Earth features and their meaning. Macmillan. 1912 (pp. T10- 
II). 
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