196 



MINERALS AND GEOLOGY 



forces; or that veins, in other words, have been filled essentially 

 from below. In this connection, it must be remembered that many 

 veins penetrate to unknown depths, and have yielded sulphurized or 

 other ores, without being yet exhausted, to the amount of thousands 

 of tons. Whilst many products found in veins are probably due in 

 part or wholly to sublimation, the great majority of these products 

 would certainly appear to have been deposited from solution : not 

 necessarily in the condition in which they now appear, but in some 

 other form from which their present condition has been derived. 

 According to certain theorists, the whole of these bodies have been 

 deposited from aqueous solution, but it is not easy to reconcile facts 

 in all cases with this assumption. Such changes and decompositions 

 as now take place in veins, lead to the conversion of many sulphur- 

 ized ores into sulphates, carbonates, and other oxidized compounds ; 

 but do not bring about, as the above hypothesis would require, 

 the conversion of these latter on the large scale into vast bodies of 

 galena, copper pyrites, arsenical pyrites, and other non-oxidized ores. 

 But if these ores, now found in such vast quanities in mineral veins, 

 really originated from soluble sulphates, chlorides, &c., the latter 

 must undoubtedly have come from some deeply-seated source ; and 

 their conversion into non-oxidized bodies could not have taken place 

 on this enormous scale without the further collaboration of endo- 

 genous agencies. 



Mineral veins are generally opened by shafts and adits, or by both 



of these methods combined. 

 In the case of veins with con- 

 siderable underlie, the shafts, or 

 openings from the surface of 

 the ground, are often carried 

 down along the slope of the 

 vein ; but in general, shafts 

 are sunk vertically, and cross- 

 cuts are carried from the sides 

 of the shaft at regular intervals 

 to the intersection of the vein. 



Galleries are then driven along FIG. loa 



the course of the latter at these points, and the sheet of ore lying 



