Published online by Cambridge University Press: 01 May 2009
A 22 m thick primitive, basanitoid sill comprises homogeneous olivine teschenite, overlain by picrite and a rhythmically layered sequence of olivine teschenite, salitic teschenite and syenite. Differentiation occurred in two stages by fractional crystallization, in part from melt residual from formation of picrite. Syenitic blebs and hydrated globules, occurring through the layered sequence, formed by the same mechanism rather than by immiscibility. Endothermic cindering of a small coal seam below the intrusion absorbed heat and steam from the sill, removing volatiles from the lower part and establishing a strong downward heat flux. The picrite was restricted to the upper part of the sill by more rapid cooling of the lower half.