THEIR CHARACTERISTICS. 103 



longest time to bring about internal change and fill with 

 crystalline and metalliferous depositions. There are, no 

 doubt, productive veins in later formations, such as the 

 lead and silver bearing veins of the Carboniferous limestone ; 

 but these by no means occur in the same richness and 

 variety as those of the primary strata. Wherever, then, 

 we have extensive developments of primary rocks and 

 mountains, as in Wales and Cornwall, Scandinavia, the 

 Ourals, the Andes and Mexican Sierras, there also we may 

 expect a corresponding development of metalliferous veins 

 gold, silver, tin, copper, and the like of varying age 

 and richness, according to certain laws the order and gover- 

 nance of which geology is yet unable to indicate. The. for- 

 mation and accumulation of rocks is in most instances a 

 slow and gradual process, but the segregation and deposition 

 of metalliferous matter is still slower, and thus we may look 

 upon the veins of the primary strata as of high antiquity, 

 though necessarily younger than the rocks they traverse. 

 Occurring most abundantly in the older formations, and 

 very rarely in those of secondary or tertiary date, it is to 

 the veins and veinstones of primary regions that the follow- 

 ing remarks will be more especially directed. 



Defining a vein as a rent or fissure in the earth's crust 

 which has been subsequently filled up by infiltrations of 

 mineral and metallic matter, it must be obvious that veins 

 will be of various widths and of various inclinations. Pro- 

 ductive veins seldom exceed a few feet in width, and it is 

 rare to find them beyond fifteen or twenty; but their in- 

 clinations are at all angles some descending almost per- 

 pendicularly, and others sloping downwards by very easy 

 stages. The bounding rocks on either side form the 

 cheeks or walls of a vein ; the mineral matter of which it 

 is composed, the vein-stuff, 'matrix, or gangue ; and the 

 metallic ore is distributed through the matrix in ribs, 



