Mk. Napier on Mineral Veins and Water- Worn Stones. 



237 



sulphur and copper. The origin of the iron pyrites vein, so regular and 

 distinct, is worthy of notice, and not easily accounted for, but by a sort 

 of polar action between the crust and kernel causing a line of demarca- 

 tion at the junction, marking the central point of the influence. 



Such veins of mundic, sulphuret of iron, in rocks, are generally ascribed 

 to heat, and more especially to sublimation, as we referred to at the com- 

 mencement of this paper; but under the circumstances in which such 

 boulders as this have been placed during the formation of this vein, no 

 such cause as heat or sublimation can be admissible. Both animal and 

 vegetable substances imbedded in the earth, and subject to the action of 

 water containing sulphate of iron, undergo decompositions ; the sulphuric 

 acid is decomposed, and sulphuret of iron formed. Thus many fossils are 

 converted into pure pyrites, but these conditions differ from this, although 

 we have heard these also ascribed to sublimation. 



We have met with many other specimens of carbonate of iron having 

 veins in them of iron pyrites, but in all cases the composition of the 

 mineral internally to the vein of pyrites, represented by the kernel, was 

 of a different composition to that part externally to the vein represented 

 by the crust, as in this second specimen, in which the analysis exhibits 

 the same character of changes as were shown in the trap rock — thus : 



Internal to pjTites vein. 



SiUca 6-4 



Protoxide of iron 48'6 



Iron pyrites 5*4 



Lime 3 2 



Magnesia 20 



Carbonic acid 32-7 



98-3 



External to pyrites vein. 



Silica 23-2 



Protoxide of iron ....37"4 



Iron pyrites 15'0 



Lime 10 



Magnesia 0"4 



Carbonic acid 22*7 



99-7 



We will refer to one other kind of boulder, or nodule, of very common 

 occurrence, formed all of iron pyrites, similar to the pyrites fossils re- 

 ferred to. Nodules of this sort are of frequent occurrence in chalk and 

 clay, many of them having, no doubt, an organic origin. The centre of 

 these nodules, although they have always a different structure from the 

 crystalline crust, differ little in composition, but when such nodules 

 are found in mineral veins, where several other metals exist, as in this 

 specimen, which is from the Parys mines, Anglesea, The centre and 

 crystalline crust are very distinct in composition, as shown in the following 



sis: — 



C'o'stalline crust. Kernel. 



Silica 1-2 



„ ., (Iron 45"G 



P^"*^« ISulphur 52-4 



99-2 



Silica 23-2 



Iron pyrites 55"2 



Galena 5"6 



Zinc blend 145 



Sulphate of lime I'l 



99-6 



