C. H. Smyth, Jr. — Replacement of Quartz by Pyrite. 279 



doubt in the case of very small grains, but with reference to 

 the general relation of quartz and pyrite in the rock, secondary 

 quartz is an entirely negligible factor. 



As to the relative amounts of interstitial filling and of 

 replacement, the evidence is quite clear. In the non-pyritiferous 

 rock, the quartz grains are very closely packed and the inter- 

 stitial filling correspondingly slight, but in the pyritiferous 

 beds the quantity of pyrite between the grains is often con- 

 siderable. The amount of replacement is greater or less accord- 

 ing as the original interstitial space was large or small. While 

 sometimes well-rounded grains are widely separated by pyrite, 

 much the commoner case is the occurrence of jagged contours, 

 and when parts of the original surfaces remain, it is evident 

 that their complete restoration would largely take up the space 

 now filled by pyrite (figs. 5 and 6). 



This is repeated so often that it is deemed excellent evidence 

 for believing that the original sand was closely packed, like 

 that of the overlying beds, with small interstitial cavities, and 

 that the considerable space now occupied by pyrite is due 

 chiefly to replacement. According to this view, figures 2, 3 

 and 4 represent later stages in a process whose earlier stages 

 are represented by figures 1 and 5. 



If the space now filled by pyrite were regarded as original, 

 it might be concluded, as there is no reason for assuming the 

 former presence of any other cement, that the sand and pyrite 

 were deposited simultaneously, the pyrite coating the grains 

 and thus keeping them from being closely packed. Primary 

 deposition of pyrite in sediments is common, an instance 

 described by Rensch* being of particular interest in this con- 

 nection ; and this explanation is favored by the apparently 

 fixed stratigraphic position of the pyrite, and the fact that it 

 was deposited before the secondary quartz. But if the fore- 

 going interpretation of the texture of the rock is correct, a 

 later introduction of the pyrite is indicated ; although, so far 

 as the evidence goes, it may have followed closely upon the 

 deposition of the sand and gravel, while they were still exposed 

 upon the sea bottom. This latter view, which is but a slight 

 modification of the preceding, would account equally well for 

 the position of the pyrite and for its deposition before the 

 secondary quartz. 



"While it is a general rule that the primary pyrite of sedi- 

 ments is deposited in the presence of abundant organic matter, 

 which is lacking in the conglomerate, the underlying Lorraine 

 formation would be a possible source of hydrogen sulphide 

 and other reducing agents. 



* Neues Jahrbucli f. Mineralogie, etc., 1879, p. 255. 



