342 Albany Museum Records. 



quartz-rock, but hard dark-brown sandstones of the older forma- 

 tions are also present. Some boulders weigh two or three hun- 

 dred weight each. Dunn also notes that many of the boulders, 

 which are perfectly rounded by attrition, are frequently deeply 

 bitten into on the surface by crystals of pyrites ; sometimes the 

 boulder is pitted all round, at others one side is more deeply eaten 

 into than the other ; where this alteration occurs the matrix is 

 highly ferruginous, and the field evidence leaves no doubt that 

 there has been actually a replacement of the silica of the boulders 

 by iron pyrites as they lay in situ in the rock. In 1896 I col- 

 lected specimens of these pitted boulders from above Vice's seam, 

 one of Dunn's localities, and they are now exhibited in the South 

 African Museum. Dunn's explanation of the chemical replace- 

 ment of silica by iron pyrites was perfectly sound, but does not 

 seem to have been made use of. The specimen that I now wish to 

 describe carries us one step further, and raises the whole question 

 of the solubility of silica at the temperature and pressure existing 

 at the earth's surface. 



The specimen, No. 45, shows a stratum of pyrites crystals, or 

 rather the casts of such crystals, for the boulder has been 

 weathered about an eighth of an inch beneath the surface, the 

 substance of the remainder of the boulder being unaffected, and 

 the original water-worn surface still in its original condition, 

 except where occasionally a pyrites crystal has pierced it from the 

 inside. We have, then, evidence for the transfusion of minerali- 

 sing waters, that is to say, there has been a suction to within the 

 boulder, through the capilliary interspaces between the quartz 

 grains, of water containing iron sulphide in solution, and there 

 has been a corresponding outflow of moisture carrying silica in 

 solution. The silica is probably dissolved and simultaneously 

 replaced by pyrites, molecule by molecule. Under such conditions 

 we can calculate the change in volume from the original quartz 

 grain to that occupied by the pyrites, according to the law stated 

 by Van Hise, (4) which is, " that the volume of the original com- 

 pound is to the volume of the compound produced directly as 

 their molecular weights, and indirectly as their specific gravities" 

 (4) A treatise on metamoiphism, 1904, p. 209. 



