Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/49

 The earliest of his geological writings which I have been able to trace is in the form of a short letter to Professor Jameson, on the occurrence of a large greenstone boulder in the Pentland Hills. It is dated from Colinton House, August 3rd, 1829, when its writer was a very little over twenty years of age. It gives an account of the position of the boulder, its composition, dimensions, and specific gravity. But the chief interest it possesses lies in the broad generalization which the young observer drew from the facts he had so carefully noted. The boulder lay upon the side of a small, steep ravine ; and its position there was such as to lead him to regard the induction as undeniable, " that the excavation of the valley must have taken place subsequently to the deposition of this boulder." He remarks further that this inference as to the lateness of the erosion of valleys is forced upon us by many other instances which intimate the gradual degradation of the soil. Those who have watched the progress of geological discussion in recent years will see at how early a period our departed friend had acquired clear views upon this subject, and had based them upon the results of actual observation. This early paper is further interesting, inasmuch as it serves to indicate the special field of geology into which Forbes's natural instincts turned him, and in which he was destined in later years to reap so abundant a harvest. He had often read and treasured in his memory the eloquent passages in which Playfair, following in the path of Hutton, had expounded the erosion of valleys and the universal decay and waste of the continents. He saw that the happy suggestions and sagacious inferences of these philosophers ought to be regarded in the light rather of an outline of what remained to be discovered than as the epitome of a completed philosophy. Whatever related to the forces which work upon the surface of the earth and effect geological changes had a special charm for him. It was this tendency which led him to wander with more than a tourist's curiosity among the glaciers of Switzerland, which first suggested to him the idea of working out by accurate observation the real cause of glacier- motion, still, in his opinion, undiscovered, and which brought him back year after year to these great mountains, where he toiled with a devotion that told at last upon his physical frame. He was the first to determine by careful measurements the amount and variations of glacier-motion. Comparing that motion to the flow of a river, he propounded the theory that " a glacier is an imperfect fluid or a viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts." The observations and journeys which led him to this deduction are detailed in his 'Travels in the Alps,' a work in which, as in the * Voyages dans les Alpes ' of De Saussure, which he took as his model, description of scenery and narrative of adventure are happily blended with scientific observation and reasoning. The vexed question of the mechanical cause of the motion of glaciers is hardly a geological problem. I would rather refer to the abundant materials collected by Forbes in this work for the elucidation of the geological functions of glaciers. The existing operations of the ice, in scoring and polishing rocks, in