Vol. 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. L^ 

 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 



