GEOLOGY. 397 



vertically one over the other, sometimes spreading in pa- 

 rallel rows to a considerable extent. In different parts of 

 Yemen, especially among the Coffee Mountains, similar 

 phenomena were observed, which contributed greatly to 

 the beauty of the landscape; particularly in the rainy sea- 

 son, 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 

 building steps to climb the hills where the ascent was dif- 

 ficult, and also as walls to support the plantations of coffee- 

 trees on the steepest declivities. The mountains south- 

 ward of Muscat, behind Ras el Hud, are chiefly of gra- 

 nite, 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 penin- 

 sula of Sinai was on the coast near Sherm. For a dis- 

 tance of about two miles the hills presented perpendicu- 

 lar 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 of lava were observed towards 

 the higher mountains, which seemed to prove that the 

 discharged matter was confined to that spot. The hills 

 round Medina, as weh 1 as the lower ridge of the great 

 northern chain, exhibit a layer of volcanic rock. It is of 

 a bluish-black colour, very porous, yet heavy and hard, 

 not glazed, and intermixed with small white substances 

 of the size of a pin-head, but not crystallized. The whole 

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

 spread. This traveller observed no lava, although the na- 

 ture of the ground seemed strongly to indicate the neigh- 

 bourhood 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 cen- 

 tury. 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, battlements, and minarets, was 

 seen ascending to heaven. The number of hot-springs 

 found at almost every station of the road to Mecca seems 

 to authorize the conjecture that similar volcanoes have 



VOL. II. 2 B 



