362 



TRANSACTIONS OF THE 



It is not only the veins the .ige of which can be de- 

 termined with respect to each other or with respect to 

 the formation which contains them^ there even exist some 

 indications of the age of metals and of the substances 

 which accompany them in the veins. 



Veins have been divided into two general orders: viz. 

 contemporaneous veins^ or those which were formed at 

 the same time as the rocks which environ them ; and true 

 veins, whose formation is considered to be subsequent to 

 that of the rocks which contain them. 



Contemporaneous veins have been usually distinguish- 

 ed from true veins by their shortness, crookedness and 

 irregularity of size, as well as by the similarity of the 

 constituent parts of the substances v^hich they contain 

 to those of the adjoining rocks, with which they are ge- 

 nerally so closely connected as to appear a part of the 

 same mass. Two other marks, more distinctive, must 

 be added. When these veins meet each other in a 

 cross direction, they do not exhibit the heaves or inter- 

 ruptions of true veins, but usually unite. In a multitude 

 of contemporaneous veins, some may appear to be heaved ; 

 but the apparent heave seldom affects more than one 

 vein, and it is in general easy to perceive that what ap- 

 pear to be separate parts of the same vein, are different 

 veins, which terminate at or near the cross vein. When 

 they meet with true veins, they are always traversed by 

 them. 



By a true vein, is understood, the mineral contents of 

 a vertical or inclined fissure, nearly straight, and of in- 

 definite length and depth. These contents are generally, 

 but not always, different from the strata, or the rocks 

 which the vein intersects. True veins have usually re- 

 gular walls,* and sometimes a thin layer of clay between 



* By this term is meant, that the rock of the country stands against the 

 vein, on each side, as a wall, without being intermixed, or forming one body 

 with it. 



