S/G6 Mr D. Forbes on a Greenstone Boulder 



little over, leaving the division which now exists between them 

 This is rendered obvious by the first of the rough outlines 

 in pen and ink which I have annexed, the correspondence of 

 the forms being there distinctly seen exactly as they appear in 

 nature, and likewise proving, by the spherical form of the frac- 

 ture, the globular structure already mentioned in the left hand 

 portion. 



The stone is excesMvely hard, except where it has begun to 

 disintegrate ; even with a very heavy hammer, it was not with- 

 out great labour that I succeeded in obtaining specimens of a 

 considerable size. I send some of the disintegrated portions, 

 which are from the concentric lamella; of the western half of the 

 boulder. You will be the fittest judge of the precise nature of 

 the stone (which appears to be greenstone, pretty highly crys- 

 tallized, and containing a little iron pyrites), and what may be 

 the most probable site of its parent rock. To one fact relating 

 to the theory of boulders, and the period at which they were 

 deposited, I would call your attention, from the peculiar situa- 

 tion in which this mass lies, and of which, in the sketches, I 

 have attempted to give some idea. Being placed upon the ac- 

 tual declivity of a small but steep ravine, it seems physically im- 

 possible that, had that valley existed at the time of its journey, 

 it should not have been precipitated to the bottom of it. If it 

 came from the last, it is not credible that it should have cross- 

 ed the channel of the streamlet, and ascended half way up the 

 western bank; and it is equally beyond explanation that, by any 

 power short of a miracle, had it come from the west, the course 

 of so enormous a mass (the weight of which I shall endeavour 

 to estimate), should have come to a stand under the influence of 

 the tremendous impetus, of whatever kind, by which it was 

 moved, in the middle of a short and steep descent of this de- 

 scription. I therefore consider the induction undeniable, that 

 the excavation of the valley must have taken place subsequent- 

 ly to the deposition of this boulder. 



In order to form a rough estimate of the size of the fragment, 

 after an attentive inspection by the eye, I conceived the western 

 portion, as far as it is uncovered, to be a semicyhnder seven feet 

 long, and three and a half in diameter ; and the eastern, a pa- 

 rallelopipcd, having a mean of six feet, four and a half feet, and 



