530 Geology and Magnetism. 



growth of which swells open the cracks, and thus causes new frac- 

 tures and dislocations, according to the variable nature of the con- 

 taining rocks, and the amount of resistance. This gradual opening 

 of the veins with the growth of the crystals from the sides accounts 

 for the isolated masses of the bounding rocks found in veins (Plate 

 XIII.), which could not possibly occur had they been open fractures. 

 Indeed the hypotheses supposing mineral veins to have been filled 

 by solution from above, or that of the injection of ingneous matter 

 into an open fissure from below, are so crude and irreconcilable 

 with the nature of their contents, that they do not deserve our at- 

 tention : the facts brought forward fully justify the conclusion that 

 all veins, whether they be mineral or not, have been formed and 

 filled on the same principle of polar action as above described. In 

 the east and west, or transverse fissures, the crystals are formed 

 from side to side, and in the splits, longitudinally, in parallel plates, 

 as shown in Plate X. The bunches of minerals in the splits are in 

 diagonal and longitudinal shoots. (Plate XV.) 



Roots and Branches of Mineral Veins. 



The meeting of a number of small veins either in depth or in a 

 horizontal direction is favourable for the accumulation of minerals. 

 Plate XVIII. represents a plan of a split vein in New Grenada 

 containing silver ores ; the feeders or roots, and the northern 

 branches, are laid down according to the manner in which the 

 mineral concentrated and dispersed northward. Many are called 

 feeders by miners which in reality have had the contrary effect. 



The feeders of the east and west veins, like those of the splits, 

 are on the south side ; the branches seen on the opposite side 

 have, generally speaking, allowed the mineral to escape from the 

 veins. An oblique fissure on the south side, called a caunter, 

 formed contemporaneously with an east and west vein, produces 

 the same effect as the small oblique branches, viz. enriching 

 one part of the east and west fracture at the expense of another ; 

 the lode north of the oblique vein would be found comparative- 

 ly poor. These facts may be observed in Cornwall in various 

 parts, and particularly in the mines of Dolcoath, Tin Croft, and 

 North Roskear. 



