This paper includes the results of a petrologic investigation of the gabbroid rocks of the San Luis Rey Quadrangle, Southern California (Fig. 1). The unusual features of these rocks were recognized by Professor E. S. Larsen during his mapping of the quadrangle, and the present study was undertaken at his suggestion for the purpose of throwing light on their origin and magmatic history.

It is a pleasure for the writer to express his grateful appreciation to Professor Charles Palache and the Holden Fund of the Department of Mineralogy, Harvard University, for financial assistance in connection with both field and laboratory expenses, and to Professor Larsen for his helpful guidance and criticism throughout the investigation. The laboratory work was carried out during the year 1933–1934 while the author was holder of the Woodworth Memorial Fellowship.

A composite batholith of Jurassic age forms a large part of the . . .

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