The Loch Doon ‘Granite’ Area, Galloway

Abstract

I. Introduction. A detailed account of these shales was written by the late Prof. H. A. Nicholson and myself, and some remarks upon their mode of deposition were made therein; but some further notes may be useful to those who are interested in the physical conditions under which sediments were accumulated. The beginning of the deposition of the Coniston Limestone marks the start of a long period of sedimentation, characterized essentially by the accumulation of fine muds with nodular bands of limestone, as noted by Prof. O. T. Jones. The Caradocian portion of the Coniston Limestone contains much detrital matter of coarse character, with interbedded limestones; but in Ashgillian times the sediments were much finer, and these fine muds continued almost uninterruptedly, save by the above-mentioned limestones, to the end of Wenlock times. There are fine grits in the Browgill division of the Stockdale Shales, and the nodular limestone-bands are almost absent in the Brathay Flags, of Wenlock age. Otherwise, conditions of sedimentation, so far as supply of mechanical material is concerned, were very similar in Ashgillian, Valentian, and Lower Salopian times; and graptolites are found in the deposits of these three periods. Details of the sediments of Ashgillian age will be found in a paper by myself, and of the Stockdale Shales in the paper to which reference has already been made. The following tabular statement summarizes the facts:— The wide distribution of these argillaceous sediments in this country and abroad, presenting generally similar characters where-ever found, indicates