C. H. /Smyth, Jr. — Replacement of Quartz by Pyrite. 277 



Art. XXY. — Replacement of Quartz by Pyrite and Corro- 

 sion of Quartz Pebbles ; by 0. H. Smyth, Jr. (With 

 Plate II.) 



I. Replacement of Quartz by Pyrite. 



The lowest beds of the Oneida conglomerate, in Central New 

 York, are characterized by the presence of considerable pyrite, 

 serving as a cement to bind together the quartz grains and 

 pebbles of which the rock is mainly composed. The relations 

 between the two minerals are worthy of note. 



The quartz is, of course, clastic, while the pyrite has been 

 deposited from solution, and would naturally be expected sim- 

 ply to fill the interstices between the more or less perfectly 

 rounded grains of the former mineral, as is the case with the 

 secondary quartz cement of the upper beds of conglomerate. 



The pyrite, however, behaves quite differently. Instead of 

 being molded upon the quartz grains and taking its shape from 

 them, the reverse relation holds good, and the form of the 

 pyrite is deeply impressed upon the quartz grains, giving them 

 angular, jagged and pitted contours, totally unlike their orig- 

 inal clastic forms. That these contours are secondary and con- 

 ditioned by the pyrite, admits of no doubt ; for not only is the 

 shape of the grains wholly incompatible with water transporta- 

 tion, but, when sections are examined in detail, it is evident 

 that the outlines of the quartz grains are commonly determined 

 by crystal faces of the pyrite, the latter mineral occurring with 

 its own forms, pyritohedron and cube, to which the older 

 quartz has been forced to accommodate itself. In other words, 

 the surfaces of the quartz are such as would have resulted had 

 it been deposited from solution upon the already crystallized 

 pyrite, which is just the reverse of the actual relation. The 

 pyrite seems to have ciwstallized as readily as though filling 

 open spaces or pushing aside some soft medium, like clay, 

 instead of forcing its way into so refractory a substance as 

 quartz ; and, in consequence, the quartz grains are bounded, 

 not by their normal, water-worn surfaces, but by variously 

 oriented minute planes, conforming to the crystal faces of 

 pyrite. Between such grains and those showing their original 

 contour there is a complete gradation, and some of the latter 

 are surrounded by pyrite. But this is decidedly exceptional, 

 the pyrite nearly always eating its way to a greater or less dis- 

 tance into the quartz, while in the absence of pyrite the quartz 

 retains its original form. 



Some idea of these various features may be gathered from 

 the accompanying photomicrographs (Plate II). Although 



