第55章

A bare, undulating tract, at the eastern end of Chatham Island, is remarkable from the number, proximity, and form of the small basaltic craters with which it is studded.They consist, either of a mere conical pile, or, but less commonly, of a circle, of black and red, glossy scoriae, partially cemented together.They vary in diameter from thirty to one hundred and fifty yards, and rise from about fifty to one hundred feet above the level of the surrounding plain.From one small eminence, Icounted sixty of these craters, all of which were within a third of a mile from each other, and many were much closer.I measured the distance between two very small craters, and found that it was only thirty yards from the summit-rim of one to the rim of the other.Small streams of black, basaltic lava, containing olivine and much glassy feldspar, have flowed from many, but not from all of these craters.The surfaces of the more recent streams were exceedingly rugged, and were crossed by great fissures; the older streams were only a little less rugged; and they were all blended and mingled together in complete confusion.The different growth, however, of the trees on the streams, often plainly marked their different ages.Had it not been for this latter character, the streams could in few cases have been distinguished; and, consequently, this wide undulatory tract might have (as probably many tracts have) been erroneously considered as formed by one great deluge of lava, instead of by a multitude of small streams, erupted from many small orifices.

In several parts of this tract, and especially at the base of the small craters, there are circular pits, with perpendicular sides, from twenty to forty feet deep.At the foot of one small crater, there were three of these pits.They have probably been formed, by the falling in of the roofs of small caverns.(M.Elie de Beaumont has described ("Mem.pour servir" etc.

tome 4 page 113) many "petits cirques d'eboulement" on Etna, of some of which the origin is historically known.) In other parts, there are mammiform hillocks, which resemble great bubbles of lava, with their summits fissured by irregular cracks, which appeared, upon entering them, to be very deep; lava has not flowed from these hillocks.There are, also, other very regular, mammiform hillocks, composed of stratified lava, and surmounted by circular, steep-sided hollows, which, I suppose have been formed by a body of gas, first, arching the strata into one of the bubble-like hillocks, and then, blowing off its summit.These several kinds of hillocks and pits, as well as the numerous, small, scoriaceous craters, all show that this tract has been penetrated, almost like a sieve, by the passage of heated vapours.The more regular hillocks could only have been heaved up, whilst the lava was in a softened state.(Sir G.Mackenzie "Travels in Iceland" pages 389 to 392, has described a plain of lava at the foot of Hecla, everywhere heaved up into great bubbles or blisters.Sir George states that this cavernous lava composes the uppermost stratum; and the same fact is affirmed by Von Buch "Descript.des Isles Canaries" page 159, with respect to the basaltic stream near Rialejo, in Teneriffe.It appears singular that it should be the upper streams that are chiefly cavernous, for one sees no reason why the upper and lower should not have been equally 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?)ALBEMARLE ISLAND.

This island consists of five, great, flat-topped craters, which, together with the one on the adjoining island of Narborough, singularly resemble 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.