The George Ashley Block (GAB), located in the Pala Pegmatite District, San Diego County, California, is a composite pegmatite-aplite dike of 8 m thickness displaying striking mineralogical layering in the aphte portion of the dike, referred to as line rock. Rhythmic layering is characterized by garnet-rich bands alternating with albite-quartz-muscovite-rich bands. Cumulus textures are notably absent from the layered portion of the dike. Elongated quartz, megacrysts are oriented perpendicular to the garnet-rich layers and poikilitically include garnet, albite, and muscovite. Calculated crystal-free magma viscosity with 3% H2O is 106.2 Pa s and the calculated settling velocity for garnet is 0??51 cm/year. Conductive cooling calculations based on emplacement of a 650??C dike into 150?? C fractured gabbroic country rock at 1??5 kbar, and accounting for latent heat of crystallization, demonstrate that the line rock portion of the dike cools to 550?? C in about 1 year. Crystal size distribution studies also suggest very rapid nucleation and crystallization. Diffusion-controlled gel crystallization experiments yield textures virtually identical to those observed in the layered aplite, including rhythmic banding, colloform layering, and band discontinuities. Thus, observed textures and calculated magmatic parameters suggest that mineralogical layering in the GAB results from an in situ diffusion-controlled process of oscillatory nucleation and crystallization. We propose that any event that promotes strong undercooling has the potential to initiate rapid heterogeneous nucleation and oscillatory crystal growth, leading to the development of a layer of excluded components in front of the crystallization front, and the formation of line rock.