Where, however, a purely local subsidence, as in Kamerun Bay and Old Calabar on the elsewhere low monotonous stretch of the Upper Guinea coast,[454] or a single great river estuary, as in the La Plata and the Columbia, affords a protected anchorage on an otherwise portless shore, such inlets assume increased importance. In the long unbroken reach of our Pacific seaboard, San Francisco Bay and the Columbia estuary are of inestimable value; while, by the treaty of 1848 with Mexico, the international boundary line was made to bend slightly south of west from the mouth of the Gila River to the coast, in order to include in the United States territory the excellent harbor of San Diego. The mere nicks in the rim of Southwest Africa constituting Walfish Bay and Angra Pequena assume considerable value as trading stations and places of refuge along that 1,200-mile reach of inhospitable coast extending from Cape Town north to Great Fish Bay.[455] It is worthy of notice in passing that, though both of these small inlets lie within the territory of German Southwest Africa, Walfish Bay with 20 miles of coast on either side is a British possession, and that two tiny islets which commands the entrance to the harbor of Angra Pequena, also belong to Great Britain. On the uniform coast of East Africa, the single considerable indentation formed by Delagoa Bay assumes immense importance, which, however, is due in part to the mineral wealth of its Transvaal hinterland. From this point northward for 35 degrees of latitude, a river mouth, like that fixing the site of Beira, or an inshore islet affording protected harborage, like that of Mombasa, serves as the single ocean gateway of a vast territory, and forms the terminus of a railroad--proof of its importance. [Sidenote: Evolution of ports.]