206 Becker — Auriferous Conglomerate of the Transvaal, 



black sands, consisting largely of magnetic iron oxide or titanic 

 iron ore. On the Rand, magnetite, though not absent, is 

 relatively rare. It does not follow that the deposits of the two 

 regions are not similar. The magnetite on the Pacific beaches 

 does not originate to any extent in the gold-quartz veins, but 

 in country rock. If the country rock happened to be poor in 

 magnetite the sands would not be black. Such seems to have 

 been the case in the Jurassic gold-bearing marine conglomerate 

 of Placer County, California. Again, the magnetite may have 

 been in part converted into pyrite by the sulphur-bearing 

 solutions accompanying the intrusion of the dikes. 



The beach sands of the Pacific Coast of North America and 

 those of JSTew Zealand form along the shores of extensive gold- 

 bearing regions. It is natural that they should so form ; it 

 would be difficult to comprehend their entire absence, and it 

 is hard enough to understand why they are not more abundant. 

 What has become of the immense quantity of gold swept away 

 by erosion from the western slope of the Sierra Nevada and 

 the auriferous territory of northern California ? It has cer- 

 tainly been carried to the sea, and, as a heavy metal, some of 

 it has been left along the coast in the beach sands. The aggre- 

 gate quantity of gold in the sand is very great, but I see no 

 probability that it represents more than a small fraction of the 

 gold which has been scattered by Nature's hydraulic process. 

 The only answer is that the metal has been so triturated, so 

 " floured," that it has been swept away into the abyss of the 

 Pacific and perhaps partly dissolved in the waters of the ocean. 

 Just as very fine gold can not be saved in a sluice, so still more 

 finely comminuted gold must remain suspended almost indefi- 

 nitely in ocean currents. 



Now 7 , at the time of the formation of the conglomerate the 

 Witwatersrand lay along the shore of an extensive auriferous 

 area, presenting belt after belt of quartz veins striking in a 

 direction nearly parallel to the coast. The drainage probably 

 cut across this area, bringing down auriferous detritus. The 

 streams certainly brought down vein quartz from a gold-bearing 

 country. This supply was more abundant at some times than at 

 others, the variation probably representing fluctuations in the 

 gradual upheaval of the land area. It is clear that vigorous 

 currents swept the shore, for otherwise such accumulations of 

 conglomerate could not have been formed. Here, ther^ there 

 were all the conditions for the formation of beach placers. It 

 might have been predicted a priori that, if littoral deposits 

 could be found within a moderate distance to the southward of 

 the area of Mashonaland, Matabeleland, and the northern 

 Transvaal, they would be nearly certain to show some alluvial 

 gold. 



