GEOLOGY OF THE SOUTH AFRICAN REPUBLIC 21 
brown powdery form that comes from precipitation. This he 
considers to have been chemically deposited as a result or 
sequence of the intrusion of igneous rocks in sheets or dikes, 
and more or less contemporaneously with the ores of silver that 
occur in these rocks. 
That gold may be carried to a considerable distance from the 
shore inthe waters ofa sea or lake, is proved by deposits recently 
examined by the writer near Denver, Col., where payable placers 
occur, at fifteen miles or more from the ancient shore line of the 
lake, in Tertiary beds made up of detritus of granitic rocks that 
form the nucleus of.the Colorado range. In this case, however, 
the gravels that contain enough gold to be worked result from 
the rearrangement and concentration of the Tertiary lake beds 
in an ancient stream bed, the goldin the undisturbed Tertiary 
beds being too small in amount to-be of economic value. 
Explorations within the conglomerate beds of the central dis- 
trict of the Rand have, as already shown, extended about one and 
one-half miles from the outcrop (which may be somewhere near 
the ancient shore line) and are still within the payable limit, 
but this does not prove that at several times that distance the 
payable limit may not have been passed. The fact, mentioned by 
Schmeisser, that the beds are generally richest at the bottom where 
the gravel is coarsest is, inso far, an evidence in favor of the placer 
theory. On the other hand many facts presented above seem 
incompatible with this theory, so that it would seem probable that 
while the beds contain gold that has been introduced mechani- 
cally, they also contain some that has been added chemically 
since the formation of the beds by concentration, either from 
adjoining sedimentary or from igneous bodies along areas that 
have been rendered accessible by dynamic movements. 
As to the other two factors, it seems abundantly proved that 
the steep dips at the outcrop do not continue in depth, but at 
what depth the auriferous conglomerates will be found in the 
middle of the basin can only be finally determined by actual 
exploration with the drill. 
The practical limits in depth at which mining can be profit- 
