in the Pentland Hills. 261 



two and a half leet, for its three dimensions. From these as- 

 sumptions, which must be considered merely as rough approxi- 

 mations, I computed the former fragment to contain about 33 

 cubic feet, and the later 67. Allowing a half of the bulk of 

 the former for the portion buried, we shall have about 117 cu- 

 bic feet for the whole. 



To o^ain some idea of the weight of the mass, I took, with 

 o-reat accuracy, the specific gravity of a fresh fractured portion, 

 which I found to be 2.90 * ; whence the weight of a cubic foot 

 will be 181 lb., and the whole mass near 200 cwt., or about 10 

 tons. Its enormous momentum may therefore be conceived, 

 when it was transported to its present situation from a distance 

 certainly of some miles. From the spontaneous decomposition 

 of the greenstone, it is not easy to determine whether it suffered 

 much from attrition in its transportation ; but I am disposed to 

 think it has not, from the angular form of the eastern fragment? 

 which now exhibits scarcely the least tendency to disintegration, 

 forming a striking contrast to the other portion with which it 

 has been so closely united. 



The argument of the excavation of valleys subsequent to the 

 deposition of boulders might, I beheve, be inferred from nume- 

 rous cases, though this is the strongest I have met with. They 

 are often placed in such insulated situations in mountainous 

 countries as to intimate the degradation of the surrounding 

 soil. 



I fear I may have extended my remarks beyond the import- 

 ance of the subject. But as you were very particular in your 

 inquiries when we last met, I collected such observations as you 

 might consider interesting. 



" Weight in air, . 7586 grains, 

 in water, 4966 



Hence sp. (jr. = ^rr^rx 77:7:7. ^= 2.895. In minute accuracy tliis should 



" 75oD — 49o0 



be increased by jjo- 



To Professor Jameson. 



