236 Mr. Napier on Mineral Veins and Water-Worn Stones. 



proper circumstances. Another boulder of a singular sort, exhibit- 

 ing the powerful action constantly going on in the earth to change the 

 character of rocks, is the specimen now shown, the crust of which is 

 of the appearance and composition of trap rock, while the kernel is 

 black, and more like a piece of shale. The crust or outer portion of 

 stones of this sort have generally been looked upon as incrustations, from 

 the water in which they have been placed having salts in solution, that 

 have become deposited upon the stone or kernel portion. Such incrusta- 

 tion does take place upon organic substances placed in mineral waters ; 

 but in this instance, as in most other minerals, the change has evidently 

 been caused by the decomposition of the original stone. The analysis of 

 the crust and kernel will show the change that has taken place in this 

 instance. 



Original Stone. Cnist. 



Insoluble 760 64-7 



Protoxide of iron 11'2 16*3 



Carbonate of lime 1'4 7"5 



Magnesia I'l 3"S 



Sulphur 3-3 3-2 



Coaly matter 5'7 0'4 



Water 15 37 



100-2 99-6 



Here we find the coaly matter disappearing, no doubt becoming oxid- 

 ised, and in all probability fixing from the water a quantity of lime 

 as carbonate. 



Another class of boulders gives a still more decided specific character, 

 and exhibits the same character of phenomena as some mineral veins, 

 and these changes evidently have taken place in the boulder since they 

 were detached from their parent rock. One or two specimens will illus- 

 trate these remarks. The first specimen is a dark crystallized carbonate 

 of iron, and has been acted upon from the surface, a vein of iron pyrites 

 marking the line of division between the crust, or part acted upon, and 

 the kernel, or portion not decomposed. 



Cnist. 



Silica 37-2 



Protoxide of iron ...30"0 



Kernel. 



Silica 6'5 



Protoxide of iron ....60'3 

 Carbonic acid 327 



99-5 



Sulphuret of iron 9*6 



Copper pyrites 08 



Carbonic acid 21-7 



99-3 



The changes here are very evident, and that they have taken place since 

 the stone became a boulder, we have only to trace the form and posi- 

 tion of the changed portion and vein of pyrites, and also that the change 

 has been caused by water penetrating the mass which has contained 



