Boulder from Molteno .Sandstone. ;U;5 



The molecular weights and specific gi-avities of the two minerals 



ai'e as follows : — 



Molecular weight. Specific gravity. 



Silica 59.94 2.G5 



Pyrites 11:. 26 5.025 



The compound proportion is therefore : — 



59.94 119.2G 



V : V< :: : = 22.6 : 23.7 



2.65 5.025 



or the volume of the pyrites is much greater than that of the silica 

 which it replaces. The formation of these pyrites crystals will 

 tend to split the boulder, and only in exceptional cases, as in the 

 Albany Museum specimen, will the outer rind of unaltered 

 quartzite remain affixed. The (_^eposition of a stratum of crystals 

 beneath the surface will, therefore, mean that in most cases the 

 outer covering will peal off and reveal the crystals as apparently 

 covering the rolled surface, and it is a point of great interest to 

 see whether this does not actually happen in the field, and that 

 Dunn's record of the boulders only being affected on the surface 

 ma3" be only true as regards their present and not their original 

 sui-face. It is hard to explain just why the crj'stals form below 

 the surface and not on it, but it is olien the case with the trans- 

 fusion of solutions that a wall is necessary to act as an interme- 

 diary in much the same way as a membrane is necessary for 

 osmosis. The special importance of the Albany Museum specimen 

 is to prove that the pyrites crystals do actually form in this posi- 

 tion. 



Since Bischof demonstrated experimentally that silicates are 

 decomposed by alkaline carbonates at ordinary temperatures and 

 pressures (5) with solution of silica the solubility of silicates 

 and quartz at tempei-atures and pressures existing at the surface 

 of the earth has become more and more recognised. We look to 

 this fact to explain the violent bends and contortions which sand- 

 stone strata assume without any sign of shattering. Examples of 

 this are well shown in the Cape Colony in the Witteberg and 



(5) Bischof. Elements of Chemical and Physical Geology, English 

 translation, 1854. 



