第56章

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, varying from the tenth of an inch to half an inch in diameter.(In the Cordillera of Chile, I have seen lava very closely resembling this variety at the Galapagos Archipelago.It contained, however, 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.) This lava, although at first sight appearing eminently porphyritic, 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 Von Buch, as projecting in great balls from the basalt of Lanzarote.("Description des Isles Canaries" page 295.) Besides the albite, this lava contains scattered grains of a green mineral, with no distinct cleavage, and closely resembling olivine (Humboldt mentions that he mistook a green augitic mineral, occurring in the volcanic rocks of the Cordillera of Quito, for olivine.); 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 surface, from the manner in which the main stream was divided by small inequalities into little rills, and especially 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 angular vesicles, have possessed little fluidity;but we see that the case has been very different at Albemarle Island.(The irregular and angular form of the vesicles is probably caused by the unequal yielding of a mass composed, in almost equal proportion, of solid crystals and of a viscid base.It certainly seems a general circumstance, as might have been expected, that in lava, which has possessed a high degree of fluidity, AS WELL AS AN EVEN-SIZED GRAIN, the vesicles are internally smooth and spherical.) The degree of fluidity in different lavas, does not seem to correspond with any APPARENT corresponding amount of difference in their composition: at Chatham Island, some streams, containing much glassy albite and some olivine, are so rugged, that they may be compared to a sea frozen during a storm; whilst the great stream at Albemarle Island is almost as smooth as a lake when ruffled by a breeze.At James Island, black basaltic lava, abounding with small grains of olivine, presents an intermediate degree of roughness; its surface being glossy, and the detached fragments resembling, in a very singular manner, folds of drapery, cables, and pieces of the bark of trees.(A specimen of basaltic lava, with a few small broken crystals of albite, given me by one of the officers, is perhaps worthy of description.It consists of cylindrical ramifications, some of which are only the twentieth of an inch in diameter, and are drawn out into the sharpest points.The mass has not been formed like a stalactite, for the points terminate both upwards and downwards.

Globules, only the fortieth of an inch in diameter, have dropped from some of the points, and adhere to the adjoining branches.The lava is vesicular, but the vesicles never reach the surface of the branches, which are smooth and glossy.As it is generally supposed that vesicles are always elongated in the direction of the movement of the fluid mass, I may observe, that in these cylindrical branches, which vary from a quarter to only the twentieth of an inch in diameter, every air-cell is spherical.)CRATERS OF TUFF.

About a mile southward of Banks' Cove, there is a fine elliptic crater, about five hundred feet in depth, and three-quarters of a mile in diameter.