The extent of this plateau from north to south, varying in our route from S.E. to S.W., is about 156 miles, or six long and seven short days' journey. Sometimes our camels went at the pace of three miles, but nearly always of two and a-half miles in the hour. It is almost impossible to make the traverse in less than fifty-six or sixty hours. The camels may continue on night and day, but it will always require so much time to make the weary journey, which is considered the greatest exploit of Saharan travelling in this portion of Northern Africa. On the road to Tuat from Algeria, or to Ghadamez from Tunis and Tripoli, or to Fezzan from Bonjem or Benioleed, there is no traverse of six days comparable in difficulty to that which we have just accomplished. There is said to be none other like it on the road to Soudan, except a tremendous desert between Ghât and Aheer. However, we must not trouble ourselves about this as yet. As for the Hamadah, we know that near Sokna the plateau breaks up and forms what are called the Jebel-es-Soudy, or Black Mountains, a most picturesque group of cliffs; and again on the route to Egypt from Mourzuk, six days' journey south-east from Sokna, it also breaks into huge cliffs, and bears the name of El-Harouj. These mountain buttresses are either the bounds of the Hamadah, or masses of rock where it breaks into hills, forming ravines or valleys. But, in fact, how far the Hamadah extends between Ghadamez on the west and Augila on the east is not yet properly ascertained. It seems to be like a broad belt intercepting the progress of commerce, civilisation, and conquest, from the shores of the Mediterranean to Central Africa. The kingdom of Fezzan, however, advances like a promontory beyond it; and then on every side stretches the desert ocean with its innumerable oases or islands, which, from being once mere fluctuating names, as it were, on a guess map, are now by degrees dropping one by one into their right places. On the breaking-up of the plateau we observed its geological structure to consist of three principal strata: first, a covering or upper crust, limestone with flints and red earth; then masses of marl; and then sandstone, lumps and masses of which were blackened by the contact of the air with the iron they contain. Under the sandstone was likewise a bed of yellow clay, with a mixture of gypsum.