The Pacific mountains begin abruptly at the south, along a generally east and west line passing some 75 miles south of the City of Mexico, where the precipitous border of the table-land of central Mexico overlooks a lower region to the south which is diversified by many volcanic mountains. But little accurate information is available concerning the geology and geography of Mexico, but in general, as is well known, there are three main mountain belts which traverse that republic. One of these mountain belts is adjacent to the Gulf of Mexico, and another is situated near the Pacific coast of the main portion of the republic, while the third forms the rugged and irregular axis of the peninsula of Lower California. Between the leading mountain belts of the mainland there are numerous short ranges and many nearly level-floored valleys. The general level of this inland region is in the neighbourhood of 6,000 feet, while the more prominent peaks and crests attain elevations of from 10,000 to 12,000 feet. The general trend of the main mountain belts, and of the numerous subordinate ranges, is about northwest and southeast. For this reason, and also because the valleys have become deeply filled with _debris_ from the mountains, travelling in directions leading north and south is facilitated by the topography, while in passing from one coast of the republic to the other the rugged bordering mountain belts have to be crossed and detours made in order to pass around the central mountain ranges. The more elevated portion of central and northern Mexico, together with the peninsula of Lower California, has an arid climate, to which many of the conspicuous features in the geography of the land are due. The precipitation over large areas is insufficient to maintain permanent streams, the vegetation is nearly all of a desert-like character, and several basins exist which do not drain to the sea. In the interior basins there are saline and alkaline lakes, and numerous dry lake beds or _playas_, which are whitened by saline efflorescences. The rocks exposed in the mountains are largely of sedimentary origin, but a characteristic feature is the geology, especially to the west, in the presence of extensive volcanic areas and lofty mountains of igneous rocks.