AMONG the many features which have lent attraction to the study of geology at Edinburgh University, Prof. Geikie's field demonstrations have always held a conspicuous place. Few favourable Saturdays have been allowed to pass, on which he might not be seen rambling with his class through some wooded glen, or climbing some rugged brae, with hammer, sketch-book, and mapcase, and every now and then stopping to point out some striking rock section, or to examine a “find,” made perchance by one of his students. But at the end of the session, when a week or ten days are devoted to the exploration of some district possessing an interesting geological structure, the “long excursion” is always looked forward to with the keenest delight by professor as well as by students. The first long class-excursion ten years ago was to Arran, and the Professor decided that his last should also be to that island—famous alike for the beauty of its scenery and for the interest attaching to its geological framework. Quarters were taken up at Corrie Hotel on Monday April 24, and that afternoon saw the whole party, numbering about a score, roaming with bags and hammers along the coast towards North Glen Sannox, and making the acquaintance of the coarse red sandstones and brecciated white quartz conglomerates of the Upper Old Red, or Lower Calciferous Sandstone series, which extend in a broad belt round that part of the island. Further inland, a coarse conglomerate made up of well-rounded pebbles of pinkish quartz interstratified with characteristic dark chocolate-coloured sandstones and occasional argillaceous beds, was ascertained some years ago by the Professor to belong to the Lower Old Red Sandstone, and to be brought down by a fault against the schists that fringe the mountainous granitic core of the northern half of the island. He had already made some progress with a geological map of the island on a scale of six inches to a mile, and he now purposes to continue this work with the co-operation of his students. Resuming his geological boundary-lines at Glen Sannox, the party was soon scouring the hillsides far and near, in search of rock-sections and exposures, while he, map in hand, remained within ear-shot, and superintended operations, marking down the lines of junction, and unravelling the geological structures with the skilful hand of one long acquainted with the art of geological mapping. In this way several miles of the boundary between the granite and schists were mapped. In the course of a walk along the steep craggy Suidhe Fearghus, on the north side of Glen Sannox, the trend of this remarkable ridge was found to coincide with that of the vertical joint in the granite, and the deep gashes which indent its profile were observed to be due sometimes to cross joints, sometimes to basalt dykes which, decomposing, have weathered down much faster than the surrounding granite. The view from Caisteal Abhail, the highest peak (2735 feet) of the ridge, was magnificent, extending southwards to Ireland, and northwards to the mountains of Mull and Arrochar. On the way down a dyke much more vitreous and obsidian-like than the other Arram pitch-stone, was crossed on the ridge between Caisteal Abhail and Cir Mhor, at the head of Glen Sannox. Another day the steps of the party were turned southwards, and as the red rocks of Glen Shurig, which runs inland from Brodick, had hitherto yielded no organic remains capable of identifying their precise geological position, the Professor instituted a methodical search, which resulted in the discovery of numerous more or less distinct impressions of the lycopod psilophyton, clearly proving them to be, as he had inferred, of Lower Old Red Sandstone age. Striking southward into Glen Dubh, the geologists then crossed the very perfect series of moraines, left there by the last valley glacier, and returning by Glen Cloy, and the well-known pitchstone dyke behind the Brodick Schoolhouse. The fossiliferous limestones and shales of Corrie were also well explored, and the position of this strata far down in the heart of the red sandstone series was remarked.