10 NEW YORK- STATE MUSEUM 



GEOLOGICAL HISTORY OF THE ORE DEPOSITS 



Clearly, there is here represented a very wide range of conditions, 

 both physical and chemical, and in any attempt to decipher the 

 histor}' of the ore deposits it is necessary to determine their relations 

 to these varx'ing conditions. If this could be done completely, the 

 genesis of the deposits would be completely explained, but as the 

 conditions, so briefly outlined, involve unlimited complexity in 

 detail, no such complete solution of the problem can be obtained. 



An obvious first step is to determine, if possible, whether the ore 

 deposits have been involved in all the changing conditions of the 

 region or in only part of them. In other words, are the deposits of 

 the same age as the limestones in which they occur and which 

 have passed through all the changes sketched above? Or, are 

 the deposits younger than the limestones and, if so, how much 

 younger, in terms of the varying conditions of the region? 



These questions are to be answered by shifting the study from 

 a consideration of the geology of the whole district to a closer 

 examination of the ore deposits themselves and, in particular, their 

 form and structural relations. 



If the ore deposits were of the same age as the limestone, they 

 would conform to the dominant original structure of the latter or, 

 in other words, the ores would occur as stratified beds or lenses, 

 intercalated parallel to the bedding of the limestone. This is 

 clearly not the case, the ores having the form of veins or irregular 

 masses which cut across the stratification of the limestone and are 

 evidently the result of deposition long subsequent to that of the 

 sedimentary series. 



The ores, then, were not formed under the conditions first 

 recorded in the geology of the region, that is, conditions of marine 

 sedimentation ; nor, obviously, could they have been formed during 

 the vastly later, second marine period, since in the latter event they 

 would be interstratified with Paleozoic sediments. 



With these periods of sedimentation eliminated, there remains to 

 be determined, if possible, the date of ore formation in terms of the 

 other stages of the geological record. 



The form of the deposits — veins and irregular masses cutting 

 across the stratification of the sedimentar}' wall rocks — is proof 

 of their deposition by underground circulation, but to fix the time 

 of deposition, other factors must be considered and particularly 

 some of the more detailed features of the deposits as related to the 

 larger geological features of the region. 



