280 C. H. Smyth, Jr. — Replacement of Quartz by Pyrite. 



Thus, so far as the pyrite itself is concerned, there is reason 

 for thinking that it was deposited in the gravel and sand lying 

 uncovered on the sea bottom. But, on the other hand, it is 

 difficult to believe that, had the pyrite been so deposited, there 

 would have been any replacement of quartz. Such metaso- 

 matic processes usually take place after rocks are deeply buried, 

 and subjected to the action of underground waters ; and while, 

 in the case of a readily soluble mineral, such as calcite, sedi- 

 mentation and replacement are often practically simultaneous, 

 with our present limited knowledge of the chemical reactions 

 on the sea bottom, it seems unlikely, in spite of the evidence 

 given later as to the solubility of quartz under surface condi- 

 tions, that this would be true of quartz. Thus, it appears 

 probable that the replacement of quartz by pyrite was effected 

 by ground waters, rather than by chemical reactions coincident 

 with, or directly following, the deposition of the sand and 

 gravel. But as this conclusion is based upon general consider- 

 ations rather than upon direct and positive evidence, it may be 

 modified by future investigation. 



As to the bearing of the stratigraphic position of the pyrite 

 upon the above conclusions, it should be said that, owing 

 to the unsatisfactory exposures of the formation, there is 

 much to be learned with reference to the precise distribution 

 of the pyrite, and it is certain that the higher beds often con- 

 tain enough of the mineral to weather yellow, brown and black. 

 It seems, therefore, unwise to lay great stress upon the strati- 

 graphic position until it is more accurately determined. 



However, even if the pyrite is largely confined to a definite 

 horizon, at the bottom of the conglomerate, it is possible to 

 reconcile the fact with the above conclusion as to its secondary 

 origin. Before cementation, the conglomerate must have 

 been a permeable formation, permitting a free circulation of 

 water. Immediately beneath it, on the other hand, the line- 

 grained Lorraine formation would be relatively impervious, 

 and thus, along the contact, or in the bottom beds of the con- 

 glomerate, the conditions would be favorable for deposition. 

 These beds might be quite uniformly impregnated with such a 

 mineral as pyrite, which, in consequence, would have a definite 

 stratigraphic position, in spite of its secondary origin. 



With so' much that is uncertain, it is not worth while to 

 make any attempt to explain in detail the chemistry of the 

 deposition of pyrite and replacement of quartz, but it is natural 

 to suppose that hydrogen sulphide and iron and alkaline car- 

 bonates were active agents in the process. These compounds 

 are sufficiently common in ground-waters to warrant the 

 assumption of their former presence where effects are observed 

 of which they would be a probable cause, although there are 



