The Rattlesnake Mountain pluton, a funnel-shaped body of coarse-grained porphyritic quartz monzonite, is an offshoot from a batholithic mass of biotite-quartz monzonite forming the central part of the San Bernardino Mountains in Southern California. The pluton intrudes a structurally complex region of metamorphic and igneous rocks that borders the batholith on the north side of the mountains. The metamorphic rocks consist of marbles, calc-silicate hornfels, schists, quartzites, amphibolites, gneisses, granulites, and migmatites. The degree of metamorphism is in the higher grades of the almandine-amphibolite facies and of the granulite facies. Igneous rocks of the pluton range from quartz monzonite through granodiorite to quartz diorite and were emplaced at the same time as the main part of the batholith. The intrusion of the pluton was postkinematic and disrupted the regional structure.
The most distinctive features of the Rattlesnake Mountain pluton are its shape and internal structure. The upper part of the funnel flares outward and is divided into several lobes and sheets separated by screens of mafic rocks. The sheets follow the funnel pattern, lying concentrically within one another and dipping inward toward a central point. The lobes are tongue-shaped and extend from the upper edge of two of the sheets. One of the lobes extends outward for more than 5 miles from the funnel. An unusual abundance of inclusions, schlieren, and dikes are present. These structures are in many cases oblique to one another; schlieren commonly cut across or are truncated by other schlieren, and inclusions lie in various orientations. Structural data were plotted on equal-area projections; in any one area, normals to the flattened surfaces of inclusions and schlieren form a pattern that commonly consists of an incomplete great circle containing a central maximum.