.414 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 26, 



more joints, and those of a more open character, than the same strata 

 lying in the valleys at a much less depth below the surface"*. The 

 jointed character of the Great Limestone in the Silver Band Mine is 

 therefore a verification of a law deduced from observations made in 

 the mining districts of Alston Moor. 



The formation of joints is not only affected by elevation, but also 

 by the position of the rocks in relation to the valleys ; that is to say, 

 where the basset of the rocks forms semicircles at the heads of valleys, 

 the rocks are less jointed than on the sides of mountains ranging in 

 a straight direction, although the elevation in each case may be equal 

 in amount. 



Most of, if not all, the minerals deposited contemporaneously with 

 the ores of lead and zinc are subjected to decomposing agents after the 

 formation of joints. The sulphate of barytes is acted upon slowly ; 

 but the limestones, which by metamorphism have become impreg- 

 nated with iron, and form the walls of the veins, decompose more 

 rapidly, and by their decomposition and removal very large caverns 

 are formed. The sulphides of lead and zinc are changed to oxides 

 and carbonates ; and occasionally the nodular masses of carbonate of 

 lead have lost their coherence and crumble into fine sand. 



If, then, the deposition of lead-ore is due to causes now in opera- 

 tion, it is evident that the causes brought into play by the formation 

 of joints counteract their effects. Indeed, from the facts connected 

 with the "Westmoreland mines, it would appear that the process of 

 the deposition of lead-ore in this district has advanced downward at 

 a slower rate than that of the formation of joints. 



The above remarks may be considered irrelevant to the subject of 

 this paper ; but I am anxious to show that the amount of decompo- 

 sition in veins and in rocks is proportionate to the amount of their 

 elevation above the sea. The progress of chemical change on mine- 

 rals in veins is undoubtedly very slow. We therefore can form no 

 adequate conception of the length of time which must necessarily 

 transpire before atmospheric agents could effect an amount of de- 

 composition in the Nenthead veins equal to that which has taken 

 place in those portions of the Silver Band veins where the sulphate 

 of barytes is not present, before the flatted or metamorphosed lime- , 

 stones in the former district, with all their contained minerals, could 

 be decomposed and carried away, forming caverns to rival in extent 

 those found in the latter mine. Yet it seems probable that, if the 

 numbers could be comprehended, they would roughly indicate a 

 period of time during the lapse of which the Penine mountains were 

 raised 1000 feet. 



In the widely ramified works made in the Nenthead veins, where 

 fewer no joints have been formed, I have found arragonite in only one 

 small cavern ; and the few specimens it contained were not of that 

 splendid arborescent kind found in the Dufton mines. Yet even in 

 the Dufton mines, where stalactitie and stalagmitic masses of lime 

 are so common, and often found highly crystalline in structure, 



* Wallace, ' The Laws which regulate the Deposition of Lead-ore in Veins, 

 &c.' 1861, p. 52. 



