n8 Galapagos Archipelago. part i. 



tenth of an inch to half-an-inch, in diameter. This 

 lava, although at first sight appearing eminently por- 

 phyritic, cannot properly be considered so, for the 

 crystals have evidently been enveloped, rounded, and 

 penetrated by the lava, like fragments of foreign rock 

 in a trap- dike. This was very clear in some specimens 

 of a similar lava, from Abingdon Island, in which the 

 only difference was, that the vesicles were spherical and 

 more numerous. The albite in these lavas is in a 

 similar condition with the leucite of Vesuvius, and with 

 the olivine, described by Yon Buch, 1 as projecting in 

 great balls from the basalt of Lanzarote. Besides the 

 albite, this lava contains scattered grains of a green 

 mineral, with no distinct cleavage, and closely resem- 

 bling olivine ; 2 but as it fuses easily into a green glass, 

 it belongs probably to the augitic family : at James 

 Island, however, a similar lava contained true olivine. 

 I obtained specimens from the actual surface, and from 

 a depth of four feet, but they differed in no respect. 

 The high degree of fluidity of this lava-stream was at 

 once evident, from its smooth and gently sloping sur- 

 face, from the manner in which the main stream was 

 divided by small inequalities into little rills, and es- 

 pecially from the manner in which its edges, far below 

 its source, and where it must have been in some degree 

 cooled, thinned out to almost nothing ; the actual 

 margin consisting of loose fragments, few of which were 

 larger than a man's head. The contrast between this 

 margin, and the steep walls, above twenty feet high, 

 bounding many of the basaltic streams at Ascension, is 

 very remarkable. It has generally been supposed that 

 lavas abounding with large crystals, and including 



1 ' Description des Isles Canaries,' p. 295. 



2 Humboldt mentions that he mistook a green augitic mineral, 

 occurring in the volcanic rocks of the Cordillera of Quito, for olivine. 



