On the Serpentine and associated Igneous Rocks of the Ayrshire Coast

Abstract

Introductory I n the twenty-second volume of this Journal is a paper by Dr. J. Geikie on the metamorphic Lower Silurian rocks of Carrick, Ayrshire. The author endeavours to prove that the rocks of the district, with some few slight exceptions, are of sedimentary origin, a “felspar porphyry” being the “maximum stage of metamorphism exhibited by the felspathic rocks of the district.” The “dioritic rocks,” “diorite and hypersthenite, both of which are occasionally foliated,” are also regarded as of metamorphic origin; and, lastly, the “compact serpentines, like the altered strata with which they are associated, are truly bedded rocks.” This view is also very clearly enunciated in the ‘Descriptive Catalogue of Rock Specimens collected by the Geological Survey of Scotland, and exhibited in the Edinburgh Museum of Science and Art’ (ed. 1870), p. 49 &c., from which I quote a few sentences. “The altered rocks of this district may be grouped into three classes—felspathic rocks, dioritic rocks, and serpentine. The felspathic rocks are by far the most abundant, and vary much in structure and aspect; the diorites and serpentine have a more definite character, and are often interbedded or closely associated. But the one class shades into the other, and no strongly marked line of division between them can be drawn upon a map. The metamorphic changes which have affected these Lower Silurian strata can hardly be exhibited in hand specimens, but must be studied in the field.” The paper referred to above was severely handled on chemical and mineralogical grounds