174 Geological Society : — Mr. Burr on Aden, 



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, previously, 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 scoriae 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- mandeb, 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 volcanic, 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 fiat 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 

 of a very cellular structure. About the middle of the east side of 

 the great crater, it contains a very thick mass, composed of alterna- 



