LEADVILLE DEPOSITS. 379 



Age of deposits. — As regards the time of deposition of the origiual ore 

 deposits, it is proved: 



That they were deposited not later than the Cretaceous period. 



That they are later than the inclosing rock is proved by their mode 

 of occurrence ; and since they have partaken of the dynamic movements to 

 which these rocks were subjected, and were folded and faulted with them, 

 they must have been formed earlier than these dynamic movements, which, 

 as the geological considerations already presented show, occurred not later 

 than the close of the Cretaceous period. 



Origin of the metallic contents. — With regard to tlic immediate source ii'om 

 which the minerals forming these deposits were derived, the following con- 

 clusions have been arrived at: 



I. Tliat they came from above. 

 II. That they were derived mainly from the neighboring eruptive rocks. 



By these statements it is not intended to deny the possibility that the 

 material may originally have come from great depths, nor to maintain that 

 they were necessarily derived entirely from eruptive rocks at present im- 

 mediately in contact with the deposits. 



The facts and reasons on which these conclusions are based will be 

 given in the following chapters. 



