410 RELATION OF METALS TO ROCKS. 



the limestone dales of the North of England mineral veins are some- 

 times directed along the master joints of the rocks, also that in the 

 slate tracts of the Craven district veins and dislocations range along 

 the cleavage planes of the slates. Dr. Boase noticed the same thing 

 in the slate tract in Cornwall, and such observations might be multi- 

 plied. Mechanical considerations would have led us to anticipate this 

 result ; for the main joints and cleavage planes are often the lines of 

 least resistance, and yield more easily to infiltration than other parts 

 of a stratum, or 1 3 any eruptive or depressing force. 



Relation of Mineral Veins to the Rocks which enclose them. 

 The relation of mineral veins to the rocks which enclose them is very 

 little understood. It is difficult to distinguish clearly between the 

 accidental and the necessary association of the phenomena of veins 

 and rock masses. 



Metalliferous veins occur more or less frequently in every class 

 of rocks, and equally in limestone, in argillaceous slate and shale, in 

 quartz and sandstone rocks, and in rocks of mingled ingredients ; in 

 uniform slates, and fragmentary millstone grit, in granular sandstone, 

 compact limestone, and crystallized limestone and granite ; in sedi- 

 mentary grits and shales, in porphyries, basalts, and metamorphic 

 conglomerates. The existence of mineral veins in a rock is therefore 

 wholly independent of the particular chemical and mineral ogical 

 nature and proximate origin of that rock ; nor, when due allowance 

 is made for the relative prevalence of the different kinds of rocks, 

 does there appear any reason to admit that any preference or more 

 frequent occurrence of metalliferous veins in rocks of particular kinds 

 can be traced, except in particular districts. There yet remains the 

 inquiry whether certain metals are specially associated with or related 

 to particular sorts of rocks. 



Diffusion of Pyrites, &c. Hardly any substance is more abundant 

 in the mineral kingdom than iron pyrites ; it occurs both in veins and 

 disseminated crystals or concretions; and, in one or other of these 

 states, it is associated with almost every known rock. It occurs dis- 

 seminated in limestone of various kinds, as crystalline limestone, 

 carboniferous limestone, and chalk ; in clay slates, shales, clays ; and 

 basalt. Veins containing iron pyrites traverse rocks of as great diver- 

 sity. Copper pyrites is not disseminated through so many rocks as 

 iron pyrites, but it occurs in veins which traverse limestone, sandstone, 

 and shale, clay slate, mica schist, granite, &c. Ores 'of manganese 

 are also very generally diffused through rocks of very different kinds. 

 The converse is true. In one and the same kind of rock occur 

 veins of copper, lead, silver, and tin. 



Relations of Metals to Slate and Granite. The tin veins of 

 Cornwall sometimes pass through clay slate and granite ; they produce 

 ores in both. A vein that has been productive of copper ore in the 

 clay state, passing into the granite, becomes richer, or, what is more 

 remarkable, furnishes ores of the same metal differently mineralized. 

 If we pursue it farther into the granite, the produce of metal is 

 frequently found to diminish. A change of ground is looked upon 



