354 MOLLUSKS OF THE GALAPAGOS STEARNS. 



equator and I (legTce soutb, are Narb()ioui»li, Albemarle, James, Inde^ 

 fatigabk', and Chatham. Of these Albcmaile is the eliief ; it is the only 

 one eut by the eciiiator, is 75 miles long and about IT) in breadth, and 

 its highest summit, aecordiiig to Humboldt, is 4,03G feet above the level 

 of the sea. Of the smaller islauds, three are between the equator and 

 1 degree south — Jervis, Dunean, and Uarrington; three between 1 de- 

 gree and 2 degrees south — Brattle, Charles, and Hood ; and live between 

 the eqnat(U- and 2 degrees north— Tower, Rindloe, Abingdon, Wennuin, 

 and Culpeiqx'r; the last only about a mile in length by five-eigliths of 

 of a mile in width. As before stated, the highest elevation oecnrs on 

 the largest island, Albemarle, 4,(>3() feet; next is Narborough, about 

 4,100; others vary in altitude from these tignres to Tower island, which 

 only reaches an elevation of about 221) feet above the sea level. 



VOLCANIC ORIGIN. 



The entire group is of volcanic origin, and most of the islands con- 

 sist of basaltic rocks and masses of scoria' and lava. "Scarcely any- 

 where else," says Humboldt in his Cosmos, " on a small space of barely 

 120 or 1 40 geographical miles in diameter, has such a countless num- 

 ber of conical mountains and extinct craters (the traces of former com- 

 munication between the interior of tlu^ earth and the atmosphere) re- 

 mained visible." Darwin, who visited the Galapagos in the expedition 

 of the BcaffJe, calculated the number of the craters at nearly two thou- 

 sand, and two of the craters were simultaneously in a state of erup- 

 tion. He wrote, " On all the islands streams of a very fluid lava may 

 be seen, which have forked off into different channels and have often 

 run into the sea." On Albemarle, " the cone nu)untains are ranged in 

 a line and consequently on lissures." " Many margins of craters are 

 formed of beds of tufa, which slope off in every direction." While 

 these islands have been regarded as of very recent formation, some of 

 them are said to exhibit the remains of an older volcanic formation; 

 these indications occur " on Charles IvSland and the small islands Gard- 

 ner, Caldwell, and Enderby, which surround it." " The structure of 

 Albemarle,* made up of a scries of at least live volcanic centers with 

 the adjacent Narborough, gives us an indication of the probable ap- 

 pearance of the central aud western groups of islands were they still 

 active so as finally to become connected and form a hnge island, with 

 James, Indefatigable, Jarvis, Duncan, liarrington, aud Cliarles as the 

 culminating points of the plateau, formed by the 100 fathom line. We 

 may therefore look upon the Galapagos Islands as a group of volcanic 

 islands, gradually bnilt np by successive flows of lava upon a huge 

 mound, itself perhaps raised by the same agencies from the floor of the 

 ocean; more active local flows in the same regiim having at special 

 points built up more rapidly the northern group of islunds — Wenman 



A. Ajjcassiz, in Hull. Mas. Comp. Zocil., Vol. xxiiij No. L 



