3995 
Canical. ‘The Disertas, lying about three leagues to the south-east 
of Madeira, Mr. Smith describes as a chain of volcanic mountains 
ranging north and south, or nearly at right angles to that of Ma- 
deira. The sea-cliffs reach to their very summits, and exhibit a 
series of beds of basalt, ashes, tufas, and volcanic brick, intersected 
by innumerable dykes. No fossils have yet been discovered on 
these islands. ; 
The occurrence of the marine limestone of San Vincente at an 
elevation of 2500 feet, proves a relative change in level of land and 
sea to that amount, previousiy, Mr. Smith is of opinion, to the ejec- 
tion of the overlying volcanic products ; but he has not observed in 
Madeira any proofs of elevation of the land during or subsequent 
to the volcanic period; though there are strong indications of sub- 
sidence, the beds of scoriz and ashes, and those containing vege- 
table remains, dipping under the sea, and occurring in situations 
where they could not have remained, had the sea level been always 
the same as at present. 
A letter, dated Madras, July 1840, addressed to John Taylor, 
Esq., Treas. G.S., by Mr. Frederick Burr, on the Geology of Aden, 
on the coast of Arabia, was afterwards read. 
The promontory of Aden, eighty miles eastward of the Straits of 
Bab-el-mandel, consists of a bold cluster of volcanic rocks with 
lofty jagged peaks, and is connected with the main land by a low 
isthmus. Its extreme length is about six miles, and its breadth is 
about three miles, and the summit of the highest point is about 
1776 feet above the level of the sea. The loftier portions of the 
promontory are wholly voleanic, and the lower are partly volcanic 
and partly consolidated sea-sand. ‘The most interesting portion of 
the district is an immense, nearly circular crater, situated at the ex- 
tremity of the promontory next the main land, and in the centre of 
which, upon a flat little raised above the sea-level, stands the town 
of Aden. ‘The diameter-of the crater is about one and a half mile, 
and it is surrounded on all sides but the eastern with precipices 
chiefly composed of lava, and rising from 1000 to 1776 feet in height. 
Although the crater appears at first sight almost perfect, Mr. Burr 
says, it has been affected by some rude shocks which have cleft it 
entirely through from north to south, forming two rents, known as 
the northern and southern passes. ‘The portion to the west of the 
fissures, and called the Gebel Shunsam, rising to the height of 1776 
feet, stands entire ; but that to the east has evidently undergone a 
partial subsidence, attaining to not more than half the height of the 
western side, and for the distance of about half a mile it has been 
broken down, allowing the sea to come almost close to the town 
and form a little bay ; but the direction of the original outline of 
the crater is indicated by the island of Seerah, situated in about the 
middle of the gap. 
To the northward of this great crater is an immense mass of lofty 
and jagged volcanic products, probably the remains of smaller 
craters. ‘ 
The prevailing rock is a dark brown or chocolate lava, generally 
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