The Tathlith one-degree quadrangle occupies an area of 11,620 sq km in the northeastern Asir region of the Kingdom of Saudi Arabia, in the southeastern part of the Precambrian shield. In the eastern part of the quadrangle the Precambrian rocks are covered by exposures of easterly-dipping sandstone of Cambrian or Ordovician age. A welldeveloped and highly integrated drainage system trending northward is worn into the Precambrian rocks, but for most of the year the wadis are dry. The Precambrian rocks of the quadrangle consist of an old, non-metamorphosed to variably metamorphosed sequence of volcanic and sedimentary rocks intruded by three main successions of plutonic and hypabyssal igneous rocks. The interlayered volcanic and sedimentary rocks occupy arcuate, north-trending fold belts in which old, rather tight north-trending folds have been refolded at least once by open folds with nearly east-trending axes. Old, northtrending left-lateral faults are associated with the fold belts and are themselves intersected by younger, northwest-trending faults. Motion on both sets of faults has been reactivated several times. An igneous differentiation suite ranging in composition from gabbro to quartz porphyry forms prominent homogeneous to zoned subcircular granitic plutons and stocks of gabbro and diorite in the older rocks. Pyroxene is present in some of the granitic rocks, and the suite itself is called the peralkalic magma series. Geochemically, the granites of the series are characterized by greater amounts of beryllium, gallium, lanthanum, niobium, lead, yttrium, and zirconium than the older granites, and magnetite from the series is typically enriched in zinc. These Precambrian rocks are overlain in the east by the Cambrian-Ordovician ferruginous Wajid Sandstone. Patches of uneroded saprolite are locally present on the surface of the Precambrian rocks under the sandstone. Earlier and more extensive weathering was a factor in supplying silt to certain valleys in the southern part of the area. Evidence for cyclic climate change is preserved in these silts. A series of interrelated research projects is needed to study the silts and other materials, including sabkhahs, Quaternary deposits, and associated anthropological, archaeological, and historic evidence, to evaluate the factors and processes of regional climate change in Arabia. Research of this type might also allow estimates to be made of the possibility for the existence of factors that could reverse the trend to regional aridity. The major ore deposit in the area is the Ash Sha'ib ancient mine for which G. H. Allcott (1970) estimated 2,170,000 tons of sulfide ore to a depth of 170 m worth $40 million for the contained zinc, copper, and silver. However, only $20 million of the ore was estimated to be worth $25 per ton or more; hence, the deposit was regarded as too small to exploit. Detailed geologic work is needed to determine if there are other similar deposits nearby. Chrysotile asbestos of good quality is present a few kilometers south of the quadrangle, an...