394 NATURAL HISTORY OF ARABIA. 



grotesque, that when seen from a distance they have the 

 appearance of volcanic mountains. Their naked perpen- 

 dicular sides present calcareous rocks, sandstone, and flint, 

 lying over each other in horizontal layers. In several parts 

 of the caravan-route between Suez and Akaba, Burckhardt 

 observed large insulated masses of porous tufwacke. The 

 famous Gebel Mokkateb, or Written Mountain, is of sand- 

 stone ; but at Wady Borak the formation changes to por- 

 phyry, alternating with strata of greenstone. 



The peninsula of Sinai exhibits a considerable diversity 

 of structures, differing in their ingredients from the ridges 

 in other parts of Arabia. On approaching the central 

 summits of the Sinai group, the traveller encounters 

 abrupt cliffs of granite from 600 to 800 feet in height, 

 whose surface is blackened by the sun. These precipices 

 enclose the Holy Mountain on three sides, leaving for a 

 passage a narrow defile about forty feet in breadth. The 

 upper nucleus, including the rugged peaks of St Catherine, 

 is composed almost entirely of granite. Among the 

 lower ridges porphyry and greenstone begin to appear. 

 In many places the latter takes the nature of slate. The 

 layers of the former are sometimes very striking, running 

 perpendicularly from the top to the base of the mountain 

 in strata of about twelve feet in width, and projecting 

 slightly from the other rocks. The porphyry of Sinai, 

 Burckhardt remarks, is usually a red indurated argillaceous 

 substance. Some specimens had the appearance of red 

 felspar. In the argil are embedded small crystals of 

 hornblende or of mica, and thin pieces of quartz, the 

 colour of which is universally red. The granite is gray 

 and of the small-grained species, of which the Towara 

 tribes manufacture hand-mills, which they dispose of to 

 the northern Arabs, or export for sale. The intervening 

 valleys are interspersed with blocks of chalk-rock ; on 

 each side, beds of red or white sandstone present their 

 smooth perpendicular surface. 



Shaw remarks, that on the route betwixt Cairo and 

 Suez an infinite number of flints and pebbles are to be 

 met with, all of them superior to the Florentine marble, 

 and frequently equal to the Mocha stone in the variety of 

 their figures and representations, having the images of 

 little trees, shrubs, or plants, impressed upon them, and 

 hence they have obtained the name of dendrite stones. 

 The porphyry of Sinai and St Catherine is distinguished by 



