Capt. Campbell on the Geognosy of the Island of Ascension. 49 
crystals of Vesuvian. Many of the rocks are in an earthy state, 
owing to the action of the weather ; and occasionally they are 
observed decaying in globular and concentric lamellar concretions. 
The upper and middle parts of the hill, marked B in the chart, 
are composed of vesicular, spumous, and corded lava. Some of 
the vesicular varieties much resemble the millstone lava of An- 
dernach. The lower part of the hill consists of rocks of a diffe- 
rent description, which form, as it were, a foundation on which 
the vesicular and corded lavas rest. On the SW. side, the rocks 
are trachyte-porphyry, occasionally including fragments of slag- 
gy lava. On the NE. side is a bluish clinkstone-lava, with nume- 
rous imbedded felspar crystals. 
It thus appears, that the Green Mountain, and the hill P, are 
composed of trachyte, and its congenerous rocks ; while B con- 
sists of vesicular and slaggy lava, resting upon trachyte. All 
those parts of the island coloured in the chart reddish-brown , are 
of the same description. The rugged parts of the island, all of 
which are coloured bluish-black in the chart, are composed of a 
greyish-black lava, slightly vesicular, and containing few crystals 
of glassy felspar. This lava presents a frightfully rugged sur- 
face, which forms irregular eminences, varying in height from 
20 to 50, and even 100 feet. 
In the bays, and on such parts of the coast as are not precipi- 
tous, the beach is formed of a sand of comminuted shells, with 
fragments of echini and of corals. In some places near to the 
sea, the fragments of shells are conglutinated together by a cal- 
careous cement, and form a pretty solid mass. The solidity of 
the mass diminishes as the distance from the sea increases. A 
turtle’s nest, with eggs, was observed imbedded in this conglo- 
merate. The rocks which rise through these calcareous beaches, 
and which are so near to the sea as to be washed by its spray, 
are incrusted with a calc-sinter and calc-tuff, formed by the ac- 
tion of the weather on the calcareous matter of the shells and 
corals. 
Lastly, it may be mentioned, that runs of a sand, composed of 
the materials of the rocks, occur in different parts of the island, 
and that these are pointed out in the chart by the pale yellow co- 
lour. 
VOL. XIV. NO. 27. JANUARY 182G. 
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