A number of laccoliths, plugs and sills of alkaline rocks, together with non-alkaline porphyritic microgranites and diorites, are intruded into Cretaceous limestones and shales in the Sierra de San Carlos. The largest intrusion, which forms the Sierra de San Jose, has a roof conformably emplaced in limestones, is probably laccolithic in form and covers about 130 km2. Some sills are up to 200 m thick but there is every gradation down to a few cm. Dykes are numerous in places and plugs from 60 to 350 m in diameter occur on the eastern side of the area. There are a few remnants of late basalt flows. Detailed mapping has not been carried out but it is known that the large Sierra de San Jose intrusion is internally complex (Bloomfield and Cepeda-Davila, 1973). This intrusion contains dioritic, pulaskitic and nepheline syenitic rocks, has a broad skarn contact zone, and is cut by late lamprophyric and phonolitic dykes. The more alkaline rocks include sodic pyroxenes and amphiboles, nepheline and analcime. The smaller intrusions are, on the whole, more alkaline and Watson (1937, p. 112) refers the plugs to ijolite, basanite and monchiquite; the dykes to camptonite, tinguaite and monchiquite; and the sills to foyaite, alkali trachyte, monchiquite and teschenite with a large sill on Cerro Sacramento being a nosean trachyte (Watson, 1937, p. 111). Detailed petrographic accounts of some dyke rocks are given by Watson (1937, p. 136) but there appears to be no detailed petrographic account of the other alkaline rocks of the area.