A second boring through the Lugar Sill

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Abstract
This paper records the details of a second boring through the Lugar Sill at Craigston House, Lugar. With the natural sections in the Bellow and Glenmuir Waters, and the Mortonmuir boring, this makes the third complete section through this remarkable intrusion. Opportunity is taken for a full comparison and correlation of the Lugar sections, and for computing on the most exact data the bulk composition of the igneous mass. The Craigston House section differs from the others in its greater proportion of ultrabasic material, and its greater alteration, the latter probably arising from the close proximity of the boring to a fault. The new section reinforces the suggestion previously made that the arrangement of the various facies within the sill is due to gravity differentiation of a picroteschenite magma at depth, followed by the injection of the resulting fractions, and possibly more of the original magma, at higher levels in one or more successive pulses.

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