Veins and nodules of cornstone, enclosed in sandstone, have reacted to the thermal influence of a Tertiary dolerite dyke by the development of a wollastonite reaction skarn. This can be shown to have formed as a result of pseudomorphous replacement of the quartz grains by wollastonite, due to outward diffusion of calcium. The process is believed to have taken place at the inversion temperature of quartz (573° C.).