GEOLOGICAL 



SOCIETY OF PENNSYLVANIA. 



361 



no theory has been advanced which would seem to re- 

 concile even a large proportion of the facts and data be- 

 fore us. However great may be the diversity of senti- 

 ment as regards the mode of their formation, and how- 

 ever great may be the difference of opinion as to their 

 positive age or to the period of their actual creation, it 

 ViTOuld appear, and justly so, that geologists (except those 

 who hold that all veins are contemporaneous and were 

 formed at the same times as the containing rocks) are 

 agreed on one important principle, viz. ihatavein which 

 is intersected, or traversed^ hy another vein ^ is older than 

 the vein by ivhich it is traversed. 



When tv^o veins thus cross each other, it is evident 

 that the one which is prolonged without interruption, 

 is of a formation more recent than that which appears 

 divided so as to allow the other to pass through it. We 

 perceive, then, in such an instance, that subsequent to 

 the formation of the first fissures, the same rocks or 

 mountains have a second time been rent in a different 

 direction along with the veins then existing, and that the 

 new fissures have received substances different from those 

 of the first. The result is that the new vein traverses 

 the old. 



There often occurs a third class of veins which cut 

 through those of the two first epochs wherever they cross ; 

 and these in their turn are traversed by those of the 

 fourth order whose formaiioii is yet more recent. 



It has been considered practicable to recognize veins 

 as belonging to eight different epochs or periods of for- 

 mation materially distinct, characterised by the metallic 

 substances which the veins contain, the state in which 

 they are found, the matrix which accompanies them, and 

 the respective disposition of the different materials. 



If we are to comprize in this grand scale of ages, the 

 veins of quartz, heavy spar, clay and earthy matters, the 

 number of these epochs would be still further multiplied. 

 I. — 2 v 



