Mr D. Forbes on a Greenstone Boulder. 259 



gas came over. About three drams of a fluid also condensed in 

 the receiver. This was poured on black oxide of manganese, 

 and again distilled. No purple tint could be observed, and a 

 little liquid which came over gave no blue colour with solution 

 of starch. 



Notice of a large Greenstone Boulder in the Pentland Hills. 

 By James D. Forbes, Esq. Communicated by the Au- 

 thor. 



CoLiNTON House, 



Dear Sib, srf August 1829. 



I NOW send you specimens of the boulder at the foot of the 

 Pentland Hills, in which, from my description, you took an in- 

 terest. It is situated at the opening of a valley named Haw 

 Dean, which lies between the hills of Allermuir and Cape Law, 

 and at a short distance behind the house of Dreghorn. By a 

 mean of two observations with the sympiesometer, on July 31. 

 and August 1. the stone lies at 333 feet above my room here, 

 which is 415 feet above the mean level of the sea, giving 748 

 feet for the elevation of the boulder above the latter point. 

 Though now in two pieces, it seems formerly to have been 

 united in one, and the mode of separation is easily explained ; 

 being composed of greenstone, with a tendency to globular de- 

 composition, the coats of one of these nuclei beginning to fail 

 from the action of the weather, the projecting part which had a 

 different concretionary structure was detached, since the equili- 

 brium of the whole mass depended greatly on the portion of the 

 nucleus, which still remains buried ; and the other portion not 

 being completely supported at its own centre of gravity, fell a 



