282 C. II Smyth, Jr. — Corrosion of Quartz. 



would be most important in determining the amount of replace- 

 ment. 



How frequently all the conditions necessary for the replace- 

 ment of quartz by pyrite are fulfilled in formations similar to 

 the Oneida conglomerate, it is at present impossible to say. 

 Judging from the literature, the matter has never been investi- 

 gated, which doubtless explains the fact that, so far as the 

 writer can learn, no case similar to the present one has been 

 described. 



Whatever sheds light upon the geological relations of so 

 important a mineral as quartz is worth recording, but it is 

 evident that this case of the replacement of quartz by pyrite is 

 of particular significance in its bearing upon ore deposits. 

 In the present instance, the total amount of replacement 

 is comparatively small, but there seems to be no reason why, 

 under more favorable conditions, there should not be a more 

 extensive, or even complete, replacement of quartz by pyrite, 

 and, perhaps, by other sulphides. Although this may be going 

 too far, it is, at least, evident that the phenomena described 

 suggest interesting possibilities. 



II. Corrosion of Quartz Pebbles. 



While the replacement of the quartz is clearly a process 

 which was accomplished early in the history of the conglom- 

 erate, the same mineral occasionally shows evidence of another 

 change, going on under very different conditions at the present 

 time. Several specimens have been found with projecting 

 pebbles of quartz deeply corroded in a manner indicating the 

 action of some solvent, working, not throughout the mass of 

 the rock, but upon surfaces of bedding planes, or, rarely, of 

 joints and boulders — in other words, where there is exposure 

 to weathering conditions. 



The pebbles show most irregular surfaces, often pitted, with 

 sharp jagged projections, and in extreme cases, appear to have 

 lost a considerable fraction of the original mass. As a rule, 

 solution has been most rapid on the tops of the pebbles, and 

 as the cement and sand work out beneath, there is also rapid 

 solution here, the result being the reduction of pebbles to 

 thin plates, with very jagged edges and rough surfaces. Such 

 a marked effect is, however, exceptional, the pebbles more com- 

 monly showing a less pronounced change of form, but having 

 the characteristic etched surfaces. The accompanying illustra- 

 tion from a photograph (fig. 1), showing a portion of the sur- 

 face of a typical specimen (natural size), gives a fairly clear 

 representation of corroded pebbles. While the sand grains are 

 similarly affected, the phenomena are, naturally, much less 

 easily observed. 



