Note on granite, porphyry or felstone

Abstract

The earliest opportunity I had of examining rocks on a large scale, was in the parish of Colvend, Stewartry of Kirkcudbright. The coast-line of this parish extends for about 12 miles, and, with some few exceptions, consists of rock from 50 to 300 feet in height, and frequently precipitous; these rocky lines consist mainly of alternate masses of aqueous and igneous rock, silurian, and what was then called porphyry. At two points in this coast-line occur beds of permian or mill-stone grit, which used to be quarried for millstones, and still supplies an excellent material for the builder. The whole of the interior of the parish is composed of syenite, rising in the north to a height of about 900 feet, and in the adjoining parish of Kirkbean to nearly 2000 feet. In no part of the parish of Colvend does this variety of granite, syenite, touch the shore, though at one point the approach is very near. There was no difficulty about the silurian rock, which is a portion of that great mass that extends from Port Patrick to Eyemouth, and occupies the chief place in the counties of Wigtown, Kirkcudbright, Dumfries, Peebles, Selkirk, and Berwick. To me the puzzle was, where did the porphyry come from which had broken through the stratified silurian at numberless points, twisting the strata into fantastic shapes, and altering their character wherever there had been contact. Gradually light dawned with the theory that granite and porphyry were mineralogically identical, but had cooled in