The circumstances that determined his bent towards the investigation of glacial geology cannot perhaps be fully elucidated now in the absence of any statement from his own pen, but it is not difficult to find many reasons that may have influenced him. In considering this subject we may glance briefly at the state of knowledge of this department of geology at the time when he began field work. The years 1861 to 1865 saw a very remarkable development of interest in glacial geology, occasioned by a sudden appreciation of the importance of many facts previously well known but imperfectly understood. Many active geologists in Scotland were coming for the first time to adopt the views which have ultimately obtained acceptance in regard to the Pleistocene history of Scotland, and the change of opinion which was going on was somewhat similar to the still greater change which took place when evolution first began to take the form of a working hypothesis or even an established law of Nature, and to sweep away a great many honoured and treasured theories that had long held sway over the minds of men. The superficial deposits of sand, clay, and stones that cover the solid rocks in the lower grounds of Scotland, often to a depth of many feet, were considered by most geologists as being of somewhat mysterious origin. It was well known that they contained boulders transported from a distance. Around Edinburgh and Glasgow, for example, large blocks of rock which must have been carried from the Southern Highlands, fifty miles or more, were familiar to those interested in geology. That the surfaces of the rocks on which the drift or boulder-clay rested were striated, grooved, and fluted, was also a well-known fact. Early in the nineteenth century a favourite explanation of these deposits, which had been supported by the celebrated Dean Buckland, was that they were the remains of the Deluge as described in Genesis. This theory, however, was soon discarded though the name “Diluvial,” still used by some writers to designate these strata, bears witness to the former acceptance of that hypothesis. For a long time they continued to be considered as flood deposits, laid down by “debacles” of obscure origin. No rational explanation for these powerful “waves of translation” could be formulated, and they failed completely to account for the remarkable scratched surfaces on which the boulder-clay rested. As the study of glaciers advanced, it became clear that moving ice, bearing debris with it, could produce striations exactly like those in question, and the boulder-clay gradually came to be considered a glacial deposit. Very important confirmation of this hypothesis came from the observations of Mr Smith of Jordanhill on the recent shelly clays of the west of Scotland. Many of the mollusca which these clays contained proved to be of species now living in Arctic seas, and the inference was obvious that at no very distant epoch a glacial climate had prevailed in Scotland.