Since this paper was prepared for the press, there has been published in the Transactions of the Royal Society of Edinburgh a description of the same place, drawn up from the joint observations of Lord Webb Seymour and Professor Playfair, Among other differences, such as may be expected to occur in the observations of different individuals, and which are not subjects for discussion, there are some which call for remark, since they involve questions of nomenclature, the discussion of which may be useful in the present uncertain state of that important preliminary to geological observation. It is much to be desired that all observers should agree in the denomination of those rocks respecting which frequent discordances arise, since without such agreement there can be no prospect of a definite application of terms.
Gneiss is described in that paper as of frequent occurrence. With a few trifling exceptions the most remarkable one of which I have described, I have no where observed any real gneiss among the schistose rocks which form the left ridge of the valley or the gear mass of stratified matter. These schistose rocks, as far as my observations go, are clay slate, mica slate, hornblende slate, and quartz rock. Gneiss is a rare rock in Scotland, and is principally to be found in the Long island, in Tirey, Coll, Rona, Iona, and Isla; as well as on the mainland, in Glen Elg, and Morven, I do not consider the rock which is of such frequent occurrence between Blair and Loch Spey, already described in this paper, as gneiss, since although the form is laminar the structure is not foliated: nor have I observed that the gneiss y of Scotland any where alternates with mica slate, although it does
