Los Pinos Mountain is located within the eastern boundary of the gabbro sub-belt in the western zone of the Peninsular Ranges batholith, southern California. It was studied in 1976 by Dr. M.J. Walawender of San Diego State University and hisl 976 paper provides the basis for this project. Los Pinos Mountain has a sub-rounded pyramid shape formed from weathering of a complex assemblage of cumulate layered ultramafic rocks within a host of hornblende gabbro. There are two distinct rock groups, the inner and outer group, which form the Los Pinos pluton. These groups are separated based primarily on mineralogy and texture. The inner group (peridotite, olivine gabbronorite, and anorthosite) is heterogeneous and consists of euhedral to subhedral cumulus of plagioclase and olivine with amphibole, ± cpx, ± opx forming the dominant post-cumulus minerals. The outer group is a more homogeneous hornblende gabbro, with hornblende typically found interstitial to euhedral plagioclase. There are also pegmatitic hornblende gab bro dikes ("comb layers") of the outer group near the contact zones with the inner group. Whole-rock geochemical analyses yield a range of SiO2 and MgO compositions from 40 to 46 wt.% and 0.14 to 27.70 wt.% for the inner group, and 43 to 49 wt.% and 4.32 to 7.24 wt.% for the outer group, respectively. Thus, this study of rocks of Los Pinos Mountain do not represent true igneous melt compositions, but are remnants of a cumulate network of deep-seated intrusions emplaced within the upper crust. A possible mechanism of emplacement is underplating of the continental arc in an extensional environment, with magma produced by melting of mafic crust and upper mantle. Differentiation of this melt by crystal fractionation in the upper crust may have produced the inner group and outer group rocks. More detailed geochemical analysis and extensive field mapping is required to better understand the spatial and temporal relationships between the Los Pinos Mountain rocks, the regional tectonics, and the Cretaceous intrusions of the Peninsular Ranges batholith.