Vor. I] STEWART—BOTANY OF THE GALAPAGOS ISLANDS 231 
squalls on Charles Island during the months of August and 
October, but none occurred on this island at the various occa- 
sions we visited it, one of which was in the early part of 
October. A thunder storm occurred around the top of Nar- 
borough Island on March 21st, being the only one seen during 
the entire year we spent among the islands. 
The effect of wind on the growth of vegetation is well shown 
on the upper parts of Charles Island, where there are several 
old tufa craters that rise from 500 to 800 ft. above the sur- 
rounding table land in the interior of the island. The northern 
sides of most of these are covered with a heavy growth of lime 
and lemon trees, on the branches of which there are mosses 
and other epiphytic plants. The southern and southeastern 
sides of these craters, on the other hand, have only low peren- 
nial herbs and bushes on them above 1350 ft., and only scat- 
tered trees for two or three hundred feet below this elevation. 
The change in the character of the vegetation is so abrupt in 
these places that the two extremes often occur within a few 
feet of each other. A somewhat similar but less pronounced 
condition of affairs is found on the upper part of Chatham 
Island, where the highest peak is covered on the leeward side 
with a thick growth of Lycopodium clavatum and _ ferns. 
Many of these are absent on the windward side, and those 
species that do persist are only a few inches in height when 
exposed directly to the action of the wind. Many species of 
lichens are found growing on the rocks and twigs on this side 
which are absent on the other. The trees of Bursera graveolens 
lean in a northwesterly direction when they are exposed to the 
wind, and their branches are often so bent and twisted as to 
give the trees much flattened crowns. 
Soil 
The substratum for the most part consists of basaltic lava, 
lava cinders, tufa, ashes, pumice, products derived from the 
disintegration of these, sand, or vegetable mold. There are 
many places in the dry regions where the lava flows are com- 
paratively recent and there is practically no soil at all. Such 
vegetation as is found there grows entirely from the crevices 
in the lava. Basaltic lava or lava approaching basalt in char- 
acter seems to form the best substratum for plants, as the 
January 16, 1911. 
