2o2 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4TH Ser. 
- densest vegetation in the dry regions, and the largest forest 
trees in the transition and moist regions, are usually found on 
lava of this kind or on soil which has been derived from it. 
On the other hand pumice forms the poorest substratum, and 
supports only low scattering bushes in places where the moist- 
ure is sufficient to support plants of a much larger size. Tufa 
makes a fairly good soil for the growth of bushes and other 
shrubby vegetation, but when forest trees occur on soil of this 
nature they are usually rather scattered and small in size. 
Where the soil is composed of ashes there usually are grassy 
areas with scattering clumps of bushes. On beds of cinders 
there is often very little vegetation of any kind, while beds of 
basaltic lava adjoining and apparently of about the same age 
may be covered with a considerable growth of plants. 
Vegetable mold only occurs in quantity in the transition and 
moist regions, the reason being that there is much more vege- 
tation in these regions to form mold, and that this vegetation 
decays very quickly owing to the larger number of fungi and 
other low organisms present. This more rapid decay of plants 
has a corresponding effect upon the disintegration of the lava, 
which takes place more rapidly than in the dry region. In his 
paper on the Galapagos Islands, Wolf mentions the great differ- 
ence in the condition of a single lava flow on the lower and 
upper parts of Charles Island. Similar conditions can be 
found on several of the other islands, notably Abingdon, Albe- 
marle, and James, on which there are lava flows the lower 
parts of which are very barren, while the upper portions are 
heavily covered with vegetation. 
Outside of the lower cryptogamic plants, certain species of 
the Cactaceae seem to be about the first plants to invade the 
recent lava in the dry regions, while some of the more xero- 
phytic species of ferns are the first in the transition and moist 
regions. Cereus nesioticus was usually found growing on 
lava, either recent or comparatively recent in origin, on which 
there were seldom any other higher plants of any size. There 
are often abrupt changes in the character of the vegetation 
on the line of contact between two different lava flows, even 
when the flows are old and both more or less heavily covered 
with vegetation. A condition of this kind is well marked on 
the sides of the mountain at Iguana Cove on Albemarle Island, 
