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262 theory, and for many years during his surveys paid no attention to it. His first recorded mention of it was a diary entry in March, 1845, recording a "jolly night at the Geological; Buckland's glaciers smashed"—the reference being to a paper read by A. F. Mackintosh on the supposed evidences of the former existence of glaciers in North Wales, controverting conclusions previously published by Buckland.

It was during the survey of the Snowdon region, where "he achieved his chief geological triumph" in unfolding the complicated history of former volcanic activity contained therein, that he began to regard the subject of glacial action seriously, and this not till he had been at work there several months. His first reference to the subject occurs in the record of a visit by Robert Chambers to him at Llanberis, in August, 1848, when Chambers and he are mentioned as having gone "out on a glacial excursion up the Pass." A walk across the hills the next day revealed "splendid examples of glacial action." The search for such examples was the special object of Chambers's visit. By the 15th of November, however, he seems to have recognized everywhere the peculiar smoothing and polishing produced by moving ice, and he described the summit of a certain precipice as being, "as usual, well grooved with glacial undulations." His first public profession of belief in the former existence of glaciers in Wales was on the occasion of an address to the Geological Society in December, 1849, on the Geological Phenomena that have produced or modified the Scenery of North Wales, in which glacial action was presented prominently. The lecturer at the same time gave new and original proofs of the former presence of glaciers, "particularly instancing cases where mountain lakes were still held back by ridges of terminal moraine, and where large blocks of rock were perched on ice-worn crags." After this his notebooks contain frequent references to glaciers. Thus he went on, meeting frequent new illustrations of the history of the Glacial period, with his eyes now opened to the existence and significance of the facts whereby he was "led to perceive the meaning of many scattered surface features in South Wales to which, at the time he was surveying in that region, he had paid little heed." On the 26th of March, 1851, he communicated his first paper on glacial phenomena to the Geological Society, On the Sequence of Events during the Pleistocene Period as evinced by the Superficial Accumulations and Surface-markings of North Wales. Having withheld this paper from publication a year for more mature study, he issued it in 1852 under the title of On the Superficial Accumulations and Surface-markings of North Wales." The chief point insisted upon in it was the prevalence of two glaciations—one widespread and prior to the deposition of the drift, the other local in the valleys and posterior to it.