From the table-land of Nejd, which has an average elevation of about 3,000 feet above the sea, there is a gradual ascent southward to the highlands of Yemen and Oman where there are mountain peaks as high as 8,000 and 10,000 feet. This diversity of surface causes an equal diversity of climate. The prevailing conditions are intense heat and dryness, and the world-zone of maximum heat in July embraces nearly the entire peninsula. On the coast the heat is more trying because of the moisture from the enormous evaporation of the land-locked basins. During part of the summer there is scarcely any difference in the register of the wet and dry-bulb thermometer. In the months of June, July and August, 1897, the averages of maximum temperature at Busrah were 100°, 103-1/2° and 102° F.; and the minimum 84°, 86-1/2° and 84° F. Nejd has a salubrious climate, while in Yemen and Oman on the highlands the mercury even in July seldom rises above 85°. In July, 1892, I passed in one day’s journey from a shade temperature of 110° F. on the coast at Hodeidah to one of 55° at Menakha on the mountains. At Sanaa there is frost for three months in the year, and Jebel Tobeyk in northwest Arabia is covered with snow all winter. In fact, all northern Arabia has a winter season with cold rains and occasional frosts. The geology of the peninsula is of true Arabian simplicity. According to Doughty it consists of a foundation stock of plutonic (igneous) rock whereon lie sandstone, and above that limestone. Going from Moab to Sinai we cross the strata in the reverse order, while in the depression of the gulf of Akaba the three strata are in regular order although again overtopped by the granite of the mountains. Fossils are very rare, but coral formation is common all along the coast. Volcanic formations and lava (called by the Arabs, harrat) crop out frequently, as in the region of Medina and Khaibar. In going by direct route from the Red Sea (Jiddah) to Busrah, we meet first granite and trap-rock, overtopped in the Harrat el-Kisshub by lavas, and further on at Wady Gerir and Jebel Shear by basalts; at the Nefud el Kasim (Boreyda) sandstones begin until we reach the limestone region of Jebel Toweyk. Thence all is gravel and sand to the Euphrates.