ANNIVERSARY ADDRESS OF THE PRESIDENT. XXXlll 



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 genera- 

 lization 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 wiU 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 indi- 

 cate 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 sugges- 

 tions 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 

 efiect 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 

 Adscous 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 i\lpes ' of De 

 Saussure, which he took as his model, description of scenery and 

 narrative of adventure are happily blended with scientific observa- 

 tion 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 Eorbes in this 

 work for the elucidation of the geological functions of glaciers. The 

 existing operations of the ice, in scoring and polishing rocks, in 



