chap. v. Fluidity of different Lavas. 117 



Albemarle Island. — This island consists of five, 

 great, flat- topped craters, which, together with the one 

 on the adjoining island of Narborough, singularly re- 

 semble each other, in form and height. The southern 

 one is 4,700 feet high, two others are 3,720 feet, a third 

 only 50 feet higher, and the remaining ones apparently 

 of nearly the same height. Three of these are situated 

 on one line, and their craters appear elongated in nearly 

 the same direction. The northern crater, which is not 

 the largest, was found by the triangulation to measure, 

 externally, no less than three miles and one-eighth of a 

 mile in diameter. Over the lips of these great, broad 

 caldrons, and from little orifices near their summits, 

 deluges of black lava have flowed down their naked sides. 



Fluidity of different lavas. — Near Tagus or Banks' 

 Cove, I examined one of these great streams of lava, 

 which is remarkable from the evidence of its former 

 high degree of fluidity, especially when its composition 

 is considered. Near the sea-coast this stream, is several 

 miles in width. It consists of a black, compact base, 

 easily fusible into a black bead, with angular and not 

 very numerous air-cells, and thickly studded with large, 

 fractured crystals of glassy albite, 1 varying from the 



affected at different times ; — have the inferior streams flowed beneath 

 the pressure of the sea, and thus been flattened, after the passage 

 through them, of bodies of gas 1 



1 In the Cordillera of Chile, I have seen lava very closely resem- 

 bling this variety at the Galapagos Archipelago. It contained, how- 

 ever, besides the albite, well-formed crystals of augite, and the base 

 (perhaps in consequence of the aggregation of the augitic particles) 

 was a shade lighter in colour. I may here remark, that in all these 

 cases, I call the feldspathic crystals, albite, from their cleavage- 

 planes (as measured by the reflecting goniometer) corresponding 

 with those of that mineral. As, however, other species of this genus 

 have lately been discovered to cleave in nearly the same planes with 

 albite, this determination must be considered as only provisional. I 

 examined the crystals in the lavas of many different parts of the 

 Galapagos group, and I found that none of them, with the exception 

 of some crystals from one part of James Island, cleaved in the 

 direction of orthite or potash-feldspar. 



