396 PROCEEDINGS OF THE ACADEMY OF [1896. 



The vegetation of the islands appears to be divided into three 

 distinguishable zones. Near the sea-level the basaltic or tufaceous 

 volcanic rocks of which the islands are exclusively composed, appear 

 almost devoid of plants, especially in the dry season, except dry 

 grayish-white, apparently dead brushwood which grows thickly be- 

 tween the blocks of ash and lava, and which on close inspection 

 exhibits inconspicuous small leaves and flowers. The most common 

 according to Wolf^ and Agassiz^ are a Verbena bush and an 

 Acacia, with an occasional tree known as the Palo Santo. Near the 

 beaches are a few species of salt loving plants, probably all identi- 

 cal, with forms also known from similar localities on the mainland. 

 Cacti, Opuntia and Cereus, are found among the blocks of lava, 

 where nothing else grows. This zone extends to a height of 800- 

 1,000 feet, the rains in general being limited even during the rainy 

 season (February or later, to July) to the higher levels above 500- 

 600 feet. The change to the second zone is sometimes very abrupt, 

 but on the leeward side of the islands the arid region extends higher 

 than on the southern side from which the moisture-bearing winds 

 come. 



The second zone is green and wooded, the Acacia and Palo Santo 

 increase in size, the Verbena disappears, and the region shows num- 

 erous open grassy spaces. The volcanic rocks, under the influence 

 of moisture, have become decomposed into a soft reddish earth. 



The last and highest region is bare of trees, having the aspect of 

 an undulating plateau covered with a rather coarse grass, Avhich ex- 

 tends to the highest summits of many of the islands. Here even in 

 the dry season, there is a more or less constant deposition of moist- 

 ure from the mists which sweep over the islands. However, both 

 above and below, on several of the islands, extremely barren local- 

 ities or areas occur of strangely desolate aspect ; in some instances 

 the arboreal vegetation of the second zone is supplemented at the 

 sea-level by thickets of mangroves or other shrubby trees, so that 

 there is, among the island floras, no absolute rule without an excep- 

 tion or two. 



The sea currents about the islands and between them and the 

 mainland are very complicated. In a general way it may be said 

 that two currents converge upon the islands, one from an east-north- 



^ EinBesuch der Galapagos Insein mit drel Kiirtchen, 1870. 

 * General sketch of the expedition of the Albatross, Feb. -May, 1891 ; Bull. 

 M. C. Zool., XXIII, No. 1. 1892. 



