502 Mr. Burr on the Geology of Aden. 



mation of recent sandstone is further interesting from its inclosing numerous ex- 

 isting shells and corals of the adjoining Arabian sea. As regards the stratification 

 of the rocks here described, the porphyries and volcanic products have generally 

 an inclination of about 1 5° from the crater, but in some places their position is 

 irregular and contorted. 



Independently of the great fissures and lines of subsidence before noticed, the 

 volcanic rocks of Aden present numerous and striking examples of dykes raking 

 through them like walls, and generally almost perpendicular. These dykes ap- 

 peared to me to be chiefly filled with siliceous matter, and to be both harder and 

 more compact than the adjoining volcanic rocks. The most prominent and remark- 

 able of them is one near the " main " or " northern pass "; it is several feet wide, 

 and has almost the appearance of a vast wall crossing the mountain. Other con- 

 spicuous dykes are seen in " Seerah " Island, cutting through both the beds of por- 

 phyry and lava. I observed in many places small veins of chalcedony intersecting 

 the lava. 



Some pieces of black and green obsidian from the volcanic rocks of this pro- 

 montory were kindly given to me by Dr. Malcolmson ; but I did not see the places 

 where they were collected, nor learn the circumstances under which they were 

 found. The green obsidian is quite glassy in its texture, and in some of the speci- 

 mens the colour is very pale. One piece is very interesting, the obsidian itself 

 running in small veins to a coarse cellular trachytic rock, and suggesting the idea 

 of materials of very unequal fusibility, having been exposed together to the intense 

 heat of the volcano, the result being a segregation of the most fusible portions, and 

 the melting of them into obsidian, while the adjacent refractory matter has under- 

 gone but little change. 



The above hasty sketch of this interesting tract is given as an imperfect outline 

 only ; but should this port be abandoned, even these rough notes may become valu- 

 able, as no European could then visit this spot. Should, on the other hand, the 

 settlement be retained, the present notice may attract the attention of future tra- 

 vellers who may have more leisure to examine the promontory of Aden. I would 

 remark, in conclusion, that few contributions to geology would be more valuable 

 than an examination not only of Aden but of the other volcanic groups, some of 

 them as yet hardly extinct, which extend north-westward from that promontory 

 through at least the southern portion of the Red Sea. 



Madras, July 1840. 



