C. H. Smyth, Jr. — Corrosion of Quartz. 285 



tion of quartz, one accompanied by simultaneous replacement 

 by pyrite, and thus metasomatic, the other purely destructive 

 in its nature, and in harmony with the general tendency of 

 weathering. While neither of these processes is of great 

 moment in the history of the formation as a whole, they are 

 interesting in that, as they seem to represent no very unusual 

 conditions, they suggest the possibility that quartz, though 

 unquestionably an exceedingly resistant mineral, may yield 

 rather more readily to the attack of natural solvents than has 

 generally been supposed. 

 Hamilton College, Clinton, N. Y. 



Description of Plate II. 



Figure 1. — Early stage of replacement, with crystals and irregular masses 

 of pyrite growing between, and into, the quartz grains. Magnified 26 

 diameters. 



Figure 2. — Original margins of quartz grains almost entirely removed, giv- 

 ing jagged contours determined by the forms of pyrite. Magnified 26 

 diameters. 



Figure 3. — Pyrite with unusually irregular forms. Some secondary quartz 

 in upper half of section. Magnified 26 diameters. 



Figure 4. — A typical example of replacement in a fine-grained specimen. 

 Nearly all the quartz grains are affected, and show the characteristic 

 contours resulting. It is quite possible that some small grains have 

 entirely disappeared. Magnified 23 diameters. 



Figure 5. — Pyrite filling interstitial spaces, and projecting slightly into 

 quartz grains, whose original margins are partly preserved. Some 

 secondary quartz. Magnified 67 diameters. 



Figure 6. — Shows very clearly the contrast between original and secondary 

 contours of quartz grains, together with the dependance of the latter 

 upon the forms of pyrite. Some secondary quartz. Magnified 67 

 diameters. 



