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.
Arabia has no rivers and none of its mountain streams (some of which are perennial) reach the seacoast. At least they do not arrive there by the _overland_ route, for it is a well-established fact that the many fresh water springs found in the Bahrein archipelago have their origin in the uplands of Arabia. At Muscat, too, water is always flowing toward the sea in abundance at the depth of ten to thirty feet below the wady-bed; this supplies excellent well-water. In fact the entire region of Hasa is full of underground watercourses and perennial springs. Coast-streams are frequent in Yemen during the rain-season and often become suddenly full to overflowing dashing everything before them. They are called _sayl_, and well illustrate Christ’s parable of the flood which demolished the house built upon the sand.
The great wadys of Arabia are its characteristic feature, celebrated since the days of Job, the Arab. These wadys, often full to the brim in winter and black by reason of frost but entirely dried up during the heat of summer, would never be suspected of giving nourishment to even a blade of grass. They are generally dry for nine and ten months in the year, during which time water is obtained from wells sunk in the wady-bed. Wady Sirhan runs in a southeasterly direction from the Hauran highlands to the Jauf district on the edge of the great Nefud; it is fed by the smaller Wady er-Rajel. Wady Dauasir which receives the Nejran streams drains all of the Asir and southern Hejaz highlands northward to Bahr Salumeh, a small lake, the only one known in the whole peninsula. The Aftan is another important wady running from the borders of Nejd into the Persian Gulf. This wady-bed is marked on some maps as a river, flowing into the Persian Gulf apparently by two mouths. It does not exist to-day. The most important water-bed in Arabia is the celebrated Wady er-Ruma, only partly explored, which flows from Hejaz across the peninsula for nearly 800 miles in a northwesterly direction toward the Euphrates. Were there a more abundant rainfall this wady would reach the Shat-el-Arab and give unity to the now disjointed water-system of Mesopotamia and north Arabia.[1] For obvious reasons the caravan routes of Arabia generally follow the course of the wadys.