on the Coast of Arabia. 501 



of commanding altitude and several miles in circumference, has evidently subsided, 

 like a cottage undermined by a neighbouring stream or river, I naturally looked 

 back to the great catastrophes which had happened on the spot, and endeavoured 

 to determine their nature. The volcano itself may probably have been submarine 

 in the first place, and successive eruptions may have gradually elevated it above 

 the level of the sea. After this its paroxysms appear to have become less violent, as 

 otherwise the sides of so vast an orifice might have attained a much greater eleva- 

 tion than 1700 or 1800 feet. From the great subsequent subsidence on the eastern 

 side, it would appear that in that direction the volcano had been gradually under- 

 mined, till at length it cracked from north to south, and the subsidence of the east- 

 ern half took place. This enormous mass, however, in subsiding seems to have 

 cracked again into three portions by fissures at right angles to the direction of the 

 principal movement. The subjoined woodcut will render this explanation more 

 clear, and make it better understood. 



Such appearing to be a brief outline of the physical history of this volcano, the 

 eruptions of which were probably accompanied by the throwing up of smaller cones 

 to the northward, I shall now briefly advert to the various rocks which compose 

 the promontory of Aden, and the phaenomena they present. The rocks which I 

 observed were the following : — A dark brown or chocolate-coloured lava, generally 

 of a very cellular texture, which is by far the most abundant rock. It forms all 

 the high peaks of the promontory, as the " Gebel Shunsam," the upper part of 

 " Seerah " Island, and the bold cluster of rocks at the northern extremity, called 

 the " Ras Marbel," and it in many places constitutes the whole mass down to the 

 sea level. This enormous accumulation of lava is sometimes, especially on the 

 east side of the great crater of Aden, separated in the middle by a very thick bed of 

 greenish porphyry, having generally a slightly lamellar structure. This porphyry 

 is itself interstratified with red ochreous beds, which occur also between it and the 

 lava, and appear to be derived from either decomposed lava, or from showers of 

 volcanic matter, afterwards converted into an ochreous clay. Near the " northern 

 pass " I observed a curious granular rock, probably a volcanic breccia ; but having 

 lost the specimens I collected, I am unable to describe it with any precision. Ap- 

 parently at the base of all the above rocks, there are in places, especially near the 

 " northern pass," thick beds of slightly consolidated sea-sand, which seem by their 

 diagonal stratification to have been drifted by opposing currents, and may probably 

 have formed the bed of the sea, when this great submarine volcano first broke forth. 

 In the northern part of the promontory, the flat line of coast, between the volcanic 

 peaks on the one side and the sea on the other, is evidently merely a raised beach, 

 where the loose sea-sand is consolidating into a tolerably compact sandstone, owing 

 to the action of a tropical sun upon the calcareous matter of the sand. This for- 



VOL. VI. SECOND SERIES. 3 T 



