chap. v. James Island. 123 



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 100 

 and 150 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 small basaltic crater. — 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 head of this promontory, a quadrant- 

 shaped segihent 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 



