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 (022-00-001), although there is a much more strongly developed contact aureole.
Ahlfeld (1966, Fig. 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.