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



affording a very unsatisfactory substitute for actual sections 

 under the microscope, they suffice to show that the relation 

 between the two minerals is quite different from that ordi- 

 narily subsisting between clastic quartz grains and their cement, 

 and can be explained only as resulting from the simultaneous 

 solution of quartz and deposition of pyrite, or, in other words, 

 the metasomatic replacement of quartz by pyrite. 



The amount of replacement varies considerably, even within 

 the limits of an ordinary thin section, and sometimes a grain of 

 quartz is considerably replaced on one side, showing the 

 jagged contour impressed by the pyrite, while the other side 

 retains the clearly defined original outline. Figures 1, 5 and 6 

 (Plate II) show cases of this kind, where the replacement is in 

 early stages and the grains retain their original outlines to some 

 extent, although the pyrite has eaten into them considerably. 

 In figures 2 and 4, on the other hand, few traces of the origi- 

 nal margins of the grains are left. 



Thus far, in spite of the variation in the amount of replace- 

 ment, no section has been seen which contains pyrite and does 

 not show replacement. Even where there are but a few scat- 

 tered crystals of pyrite, they have eaten into the adjacent 

 quartz just as in parts of the rock that are strongly pyritifer- 

 ous (fig. 1). Sometimes such isolated crystals, and even 

 larger masses of pyrite occur entirely within quartz grains, 

 having, doubtless, been deposited from solutions entering 

 cracks. It is evident, however, that, as a rule, the replacement 

 started at the surfaces of the grains. 



This is a matter of interest in its bearing upon the history of 

 the replacement, as it shows plainly that the solutions effecting 

 the change circulated through the beds of sand and gravel 

 before they were cemented. For, had these beds been indu- 

 rated, as the upper beds are now, by secondary quartz, filling 

 the interstices between the grains, any replacement that might 

 have followed would have been controlled by secondary sur- 

 faces, resulting from fractures subsequent to cementation, and 

 would show no such dependence upon the original surfaces of 

 the quartz grains. 



Such openings as escaped complete filling by pyrite were 

 subsequently dosed by secondary quartz. It is evident that 

 when this happened the quartz would take the form of the 

 pyrite, and thus give a texture identical in appearance with 

 that described above. This relation is shown in figures 3, 5 and 

 6, and may be seen frequently in the sections, but it is always 

 a minor feature ; and while, in photographs with low powers, it 

 might be confused with replacement, when seen under the 

 microscope, the results of the two processes are easily distin- 

 guished, as they are in figures 5 and 6. There might be some 



