A thirty- three square mile area, east of La Mision de San Miguel, Baja California, was mapped on air photos at a scale of 1:35,700, The oldest formation is Mesozoic volcanic flow rock interbedded with well indurated epiclastic tuff breccias. These rocks are tilted at moderate angles to the southwest. An elongate one and one half mile long pluton of quartz diorite has intruded the volcanic and epiclastic rocks. Both Mesozoic volcanic and nlutonic rocks are unconformably overlain by the Late Cretaceous Rosario Formation consisting of calcareous subgraywacke interbedded with mudstone and sandstone, with leaser proportions of conglomerate. The Rosario Formation is overlain in succession by Miocene(?) vesicular and porphyritic basalts, Miocene tuffaceous sandstone and marine feldspathic sandstone, and breccia which is either part of the Miocene sequence, or younger. The marine sandstone contains a twelve foot layer of diatoma.ceous earth. There are remnants of a stream terrace 50-100 feet above the present stream level. Fossils of the Rosario Formation include e.mmonites, baculites, echinoderm spines, three gastropod genera, nine pelecypod genera and a brachiopod. Undescribed fossils include limpets, and Lima-like clams. Miocene fossils include diatoms, rad.iolarians, the foraminifera Orbulina !12.•, other planktonic foreminifera, shark$ 1 teeth, mammal and fish bones, the pelecypod Chione (Chionopsis) tembloreneis?, and the gastropods Turritella oooyana and Terebra (Strioterebrum)? sp.