GEOLOGY. 355 



«ter, and so uniformly regular, that they might be mistaken 

 for the work of art. They rise vertically one over the other, 

 sometimes spreading in parallel rows to a considerable extent. 

 In different parts of Yemen, especially among the Coffee 

 Mountains, similar phenomena were observed, which contrib- 

 uted greatly to the beauty of the landscape ; particularly in 

 the rainy season, when the water was seen rushing over their 

 summits, and forming cascades, which had the appearance 

 of being supported by rows of artificial pillars. These basalts 

 were useful to the inhabitants, serving as materials for build- 

 ing steps to climb the hills where the ascent was ditficult, and 

 also as walls to support the plantations of coffee-trees on the 

 steepest declivities. The mountains southward of Muscat, 

 behind Ras el Hud, are chiefly of granite, and according to 

 Captain Owen rise to the height of 6000 feet. 



Volcanic Rocks. — The first and only appearance of volcanic 

 action which Burckhardt detected in the peninsula of Sinai 

 was on the coast near Sherm. For a distance of about two 

 miles the hills presented perpendicular cliffs from sixty to 

 eighty feet in height, some of them nearly circular, others 

 semicircular. The rocks were black, slightly tinged with 

 red, of a rough surface, and full of cavities. In other places 

 there was an appearance of volcanic craters. No traces o* 

 lava vvere observed towards the higher mountains, which 

 seemed to prove that the discharged matter was confined to 

 that spot. The hills round Medina, as well as the lower 

 ridge of the great northern chain, exhibit a layer of volcanic 

 rock. It is of a bluish-black colour, verv"^ porous, yet heavy 

 and hard, not glazed, and intermixed with small white sub- 

 stances of the size of a pin-head, but not cr)^stallized. The 

 whole plain is blackened by the debris, with which it is over- 

 spread. This traveller observed no lava, although the nature 

 of the ground seemed strongly to indicate the neighbourhood 

 of a volcano. The inhabitants gave him an account of an 

 earthquake and a volcanic eruption, which took place there 

 about the middle of the thirteenth century. They described 

 it as bursting forth eastward of the town, with a smoke that 

 completely darkened the sky ; at the same time a fiery mass 

 of immense size, resembling a large city with walls, battle- 

 ments, and minarets, was seen ascending to heaven. The 

 number of hot-springs found at almost every station of the 

 iroad to Mecca seems to authorize the conjecture that simi- 



