The topography of this region has been quite thoroughly described by W. A. Goodyear, in former reports of the State Mining Bureau. The structure of San Diego County is comparatively simple. Three main divisions might be made: the desert on the east, the Peninsula range of crystalline rocks in the middle, and the nearly level mesa on the west. The Peninsula range is supposed to represent the southern continuation of the Sierra Nevadas, but in just what relation it stands to the Sierras has been a matter of dispute. The Peninsula range in San Diego County forms one main mountain chain. It maintains this simplicity of structure southward, forming the backbone of the peninsula of Lower California. Northward it becomes broader and more complex, rising in the lofty San Jacinto and San Bernardino ranges on the east, and the Santa Ana range on the west, while the region between is filled with mountains and valleys irregularly disposed. Complex as is the topography of this region, the geological problems, though often difficult to solve, are quite simple. The higher mountains are formed wholly of ancient crystalline schists and massive rocks, respecting the age of which a great diversity of opinion has existed; while the region bordering the coast consists of unaltered Cretaceous, Tertiary, and Quaternary deposits. Owing to the very limited time given me to prepare my field notes for the press, they will be given substantially as they were taken in the field, without any attempt at systematic arrangement. The crystalline rocks of San Diego County are varied in character, and of much interest. No opportunity has been given me to study the large collection made, and the determinations given are simply the result of superficial examination, and are subject to correction. The bay of San Diego is bordered on the east by gently sloping mesas of modern Tertiary and Quaternary age. These unaltered strata are characteristic of the western slope of the Peninsula range through its whole extent. They sometimes rise as high as 3,000 feet; though in San Diego County they do not exceed 1,500 feet. The upper portion of these beds consists to a great extent of coarse, loosely cemented conglomerates. The rivers issue from the higher mountains through narrow valleys or cañons, and have cut valleys, often quite broad and with very steep sides, through the mesas to the ocean. The Otay mesa has a height of about 500 feet, the western portion being somewhat higher than the eastern, indicating a recent elevation near the coast. The soil of the mesa is adobe, due to the decay of porphyry mountains to the east. Under the adobe there is a calcareous marl, often many feet thick.