This intrusion of about 1.2x0.4 km has a narrow, hornfelsed aureole 15-20 m wide. One rock type consists of 70% nepheline, which is slightly replaced by cancrinite, a little orthoclase and albite, and sparse biotite and sodic amphibole; femics constitute less than 10% of the rock.
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The size of this intrusion has not been determined accurately because of the high relief and dense vegetation, but it is smaller than Cerro San Cristobal (No. 1), although there is a much more strongly developed contact aureole.
Ahlfeld (1966, Fig. 1_10) on his sketch map shows this occurrence to consist of two intrusions, the larger southern one being an easterly-dipping sheet extending north-south over 4 km. There is an extensive contact aureole, particularly on the eastern side, with veining and brecciation.
A phonolite flow occurring north-northeast of San Pedro is intercalated in Carboniferous calcareous sediments.