Boulder from Molteno Sandstone. 345 



we find preciselj- pyritic nodules which, where the quartz pebbles 

 are large, are of the same size, and where the banket is gravelly, 

 the nodules become of buck-shot size. Drs. Hatch and Corstor- 

 phine ascribe the origin of these pyritic nodules to growth by 

 accretion, but the authors do not explain how the space for their 

 growth was produced (10). The more probable explanation of 

 these pyritic nodules is that they are quartz pebbles replaced by 

 pyrites in the same waj- as they are replaced elsewhere by calcite. 

 It is remarkable that these pyritic nodules are said by the above 

 authors to consist of marcasite, a mineral identical in composition 

 with pyrites, but the crystalline form of which occupies 8 per 

 cent, more space than pyiites ; seeing the increase of space 

 required for the replacement of (juartz by pyrites, it seems 

 unaccountable that the iron sulphide should assume a crystalline 

 form which requires still greater space. 



Finally, we have to trace what becomes of the silica that has 

 gone into solution, and an examination of the sand grains of the 

 sandstone matrix at once reveals the secret. The Molteno sand- 

 stones generally are characterised by the great brilliancy they 

 possess when the sun shines on them, a fact noted by Darwin in 

 sandstones of the same age in Australia ; from this fact Dunn 

 called them " glittering sandstones," If a few grains are put 

 under the microscope, some of them will show perfect crystalline 

 faces, and on a closer examination it will be seen that inside the 

 quartz crystal there is the original sand grain, rounded, and with 

 all its little quota of dirt still adherent. In other words, the silica 

 from solution has been deposited round the sand-grains, and has 

 built up new, perfect crystals on the old foundations, and it is the 

 faces of these rejuvenated crystals that glint and shine on the 

 rock surface. 



(10) The Petrography of the Witwatersrand Conglomerate, Trans, 

 S.A. Geol. Soc, Vol. VII, p. 141. 



