xiv.J FORBES* SCIENTIFIC WORK. J87 



he happened to be studying were discussed, or through 

 which collateral information might be gathered. 



' Another of his early contributions to geology is in 

 the form of a short letter to Professor Jameson, on the 

 occurrence of a large greenstone boulder in the Pentland 

 Hills. 1 It is dated from Colinton House, 3d August, 

 1829, when its writer was a little over twenty years of 

 . It gives an account of the position of the boulder, 

 its composition, dimensions, and specific gravity. But 

 chief interest it possesses lies in the broad general i- 

 zation 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 

 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 

 mces, 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, in- 

 asmuch as it serves to indicate the special field of geology 

 which Forbes' natural instincts turned him, and in 

 which lie was destined in later years to reap so abundant 

 a harvest. He had often read, and treasured in his 

 memory, the eloquent ] - in which Playfair, follow- 



11 the path of Hutton, had expounded the erosion of 

 ys, and the universal decay and waste of the conti- 

 nents. He saw that the happy suggestions and sagacious 

 inferences of these philosophers ought to le regarded in the 

 ther <f an outline of what remained to IK- dis- 

 tlian as the epitome of a completed philosophy, 

 lated to the forces which work upon the 

 u-tli and effect geological changes had a 

 harm for him. It was this tendency which led 

 i / 



