358 GALAPAGOS AECHIPELAGO. [CHAP. xvn. 



the Equator, and between five and six hundred miles westward of 

 the coast of America. They are all formed of volcanic rocks ; a fe\v 

 fragments of granite curiously glazed and altered by the heat, can 

 hardly be considered as an exception. Some of the craters, sur- 

 mounting the larger islands, are of immense size, and they rise to 

 a height of between three and four thousand feet. Their flanks are 

 studded by innumerable smaller orifices. I scarcely hesitate to 

 affirm, that there must be in the whole archipelago at least two 

 thoitsand craters. These consist either of lava and scorise, or of 

 finely-stratified, sandstone-like tuff. Most of the latter are beauti- 

 fully symmetrical ; they owe their origin to eruptions of volcanic 

 mud without any lava : it is a remarkable circumstance that every 

 one of the twenty-eight tuff-craters which were examined, had 

 their southern sides either much lower than the other sides, or 

 quite broken down and removed. As all these craters apparently 

 have been formed when standing in the sea, and as the waves from 

 the trade wind and the swell from the open Pacific here unite their 

 forces on the southern coasts of all the islands, this singular uni- 

 formity in the broken state of the craters, composed of the soft and 

 yielding tuff, is easily explained. 



Considering that these islands are placed directly under the 

 equator, the climate is far from being excessively hot ; this seems 

 chiefly caused by the singularly low temperature of the surrounding 

 water, brought here by the great southern Polar current. Except- 

 ing during one short season, very little rain falls, and even then it 

 is irregular; but the clouds generally hang low. Hence, whilst 

 the lower parts of the islands are very sterile, the upper parts, at 

 a height of a thousand feet and upwards, possess a damp climate 

 and a tolerably luxuriant vegetation. This is especially Ihe case 

 on the windward sides of the islands, which first receive and 

 condense the moisture from the atmosphere. 



In the morning (17th) we landed on Chatham Island, which, 

 like the others, rises with a tame and rounded outline, broken 

 here and there by scattered hillocks, the remains of former craters. 

 Nothing could be less inviting than the first appearance. A broken 

 field of black basaltic lava, thrown into the most rugged waves, 

 and crossed by great fissures, is everywhere covered by stunted, 

 sun-burnt brushwood, which shows little signs of life. The dry 

 and parched surface, being heated by the noon-day sun, gave to 

 the air a close and sultry feeling, like that from a stove: we fancied 



