第58章

Two craters of tuff on this island are the only remaining ones which require any notice.One of them lies a mile and a half inland from Puerto Grande: it is circular, about the third of a mile in diameter, and 400 feet in depth.It differs from all the other tuff-craters which I examined, in having the lower part of its cavity, to the height of between one hundred and one hundred and fifty feet, formed by a precipitous wall of basalt, giving to the crater the appearance of having burst through a solid sheet of rock.The upper part of this crater consists of strata of the altered tuff, with a semi-resinous fracture.Its bottom is occupied by a shallow lake of brine, covering layers of salt, which rest on deep black mud.The other crater lies at the distance of a few miles, and is only remarkable from its size and perfect condition.Its summit is 1,200 feet above the level of the sea, and the interior hollow is 600 feet deep.Its external sloping surface presented a curious appearance from the smoothness of the wide layers of tuff, which resembled a vast plastered floor.Brattle Island is, I believe, the largest crater in the Archipelago composed of tuff; its interior diameter is nearly a nautical mile.At present it is in a ruined condition, consisting of little more than half a circle open to the south;its great size is probably due, in part, to internal degradation, from the action of the sea.

SEGMENT OF A BASALTIC CRATER.

(FIGURE 14.SEGMENT OF A VERY SMALL ORIFICE OF ERUPTION, on the beach of Fresh-water Bay.)One side of Fresh-water Bay, in James Island, is bounded by a promontory, which forms the last wreck of a great crater.On the beach of this promontory, a quadrant-shaped segment of a small subordinate point of eruption stands exposed.It consists of nine separate little streams of lava piled upon each other; and of an irregular pinnacle, about fifteen feet high, of reddish-brown, vesicular basalt, abounding with large crystals of glassy albite, and with fused augite.This pinnacle, and some adjoining paps of rock on the beach, represent the axis of the crater.The streams of lava can be followed up a little ravine, at right angles to the coast, for between ten and fifteen yards, where they are hidden by detritus: along the beach they are visible for nearly eighty yards, and Ido not believe that they extend much further.The three lower streams are united to the pinnacle; and at the point of junction (as shown in Figure 14, a rude sketch made on the spot), they are slightly arched, as if in the act of flowing over the lip of the crater.The six upper streams no doubt were originally united to this same column before it was worn down by the sea.The lava of these streams is of similar composition with that of the pinnacle, excepting that the crystals of albite appear to be more comminuted, and the grains of fused augite are absent.Each stream is separated from the one above it by a few inches, or at most by one or two feet in thickness, of loose fragmentary scoriae, apparently derived from the abrasion of the streams in passing over each other.All these streams are very remarkable from their thinness.I carefully measured several of them; one was eight inches thick, but was firmly coated with three inches above, and three inches below, of red scoriaceous rock (which is the case with all the streams), making altogether a thickness of fourteen inches:

this thickness was preserved quite uniformly along the entire length of the section.A second stream was only eight inches thick, including both the upper and lower scoriaceous surfaces.Until examining this section, I had not thought it possible that lava could have flowed in such uniformly thin sheets over a surface far from smooth.These little streams closely resemble in composition that great deluge of lava at Albemarle Island, which likewise must have possessed a high degree of fluidity.

PSEUDO-EXTRANEOUS, EJECTED FRAGMENTS.