Vor. I] STEWART—BOTANY OF THE GALAPAGOS ISLANDS 221 
the fog banks which strike the windward sides of the mountains 
at various elevations, whereupon the fog is thrown down as 
fine mist and sometimes rain. These fog banks, however, do 
not always extend to the tops of the mountains, as these are 
often clear while the region a few hundred feet below may 
be entirely enveloped in fog. The soil at the tops of the 
mountains is. sometimes dusty, while a little below the top it 
may be very moist or even muddy. From February until June 
inclusive there is much less fog in the upper regions than 
during the remainder of the year. During these four or five 
months the tops of the mountains may be entirely clear for 
several days at a time; but during the remainder of the year 
they are enveloped in fog, with only occasional clear days. It 
sometimes happens that the fog will clear away in the early 
evening to reappear again the following morning. 
The direct effect of the fog on the growth of vegetation is 
well shown on some of the islands, especially so on Duncan 
above 1000 ft. elevation. The south sides of many of the large 
lava boulders here are covered with a heavy growth of Poly- 
podium squamatum, while the other sides are entirely bare. 
This condition is due to the fact that the southern exposures 
are more directly bathed by the fog-laden wind than are the 
others. Such instances as this are rather common; the wind- 
ward sides of trees and bushes often have a heavier growth of 
epiphytic lichens and mosses than the leeward sides. 
Streams and springs of water are very scarce on the islands, 
in fact entirely absent on most of them. There are several 
springs on Chatham Island above 1000 ft. elevation, one of 
them large enough to supply a sugar mill as well as all the 
various needs of a population of some three hundred. There 
is also on this island a crater lake of considerable size and 
depth. Furthermore a few small streams occur in the upper 
regions of this island, but as they are mostly fed by surface 
water they quickly dry up as soon as the rainy season is over. 
Charles Island has two springs of fair size, and several seep- 
ages of water around the base of a tufa crater at 1000 ft.; 
but none of these affords sufficient water to form more than 
a small brook that sinks from sight a short distance away from 
its source. There are also several small basins in the plateau 
region of this island around 1000 ft. elevation, but they were 
