The Long Potrero pluton is a zoned circular feature whose composition grades inward from pyroxene-bearing biotite-hornblende tonalite to biotite-muscovite leucotonalite. It is part of a much larger composite tonalite body whose margins are not fully defined. Textural and structural data indicate that the pluton was emplaced by diapiric upwelling. This diapirism was accompanied by differentiation involving plagioclase, hornblende, and to a limited extent biotite. This produced a more basic (i.e. less-differentiated) head on the diapir and more felsic tail. As the magma intruded to the present erosional level, the differentiated tail of the diapir caught up with, and intruded, its own head forming the zoned structure. No evidence for post plutonic deformation of the pluton could be found. This tonalitic magma did not differentiate to form granodioritic rocks and suggests that a separate parent melt is necessary for the production of these more silicic plutons in the Peninsular Ranges batholith.