232 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, 



