[TYRRELL] THE GOLD OF THE KLONDIKE 35 
These smaller streams continued their downward erosion, and the 
formation of V-shaped valleys, until the larger streams into which they 
flowed had reached base level, and then they began the process of lateral 
planation, and the formation of flood-plains, although the sides of 
the valleys were still steep and rocky, and they presented a young rather 
than a mature aspect. 
The gravel in the flood-plains of these new valleys is rarely deep, 
and the material of which it is composed is partly derived from the wear- 
ing and breaking down of the adjoining rocks, and partly from the older 
gravels of the White Channel Period. 
These gravels of the Third Cycle of Erosion are of Pleistocene age 
and contain a large number of bones of mammals, some of which are now 
extinct. The most abundant are those of a bison, Bison crassicornis 
and the mammoth, Elephus primigenius, while the following species have 
also been recorded:—Bison occidentalis, Bootherium bombifrons, Masto- 
don americanus, Ovibos moschatus, Symbos Tyrrelli, Cervus Canadensis, 
Equus sp., Ovis sp., Alce sp., Rangifer sp., Arctotherium yukonense, Ursus 
sp., Canis sp. 
The net result of the erosion accomplished during the Second and 
Third Cycles has therefore been that a moderately mature style of 
topography was first formed, with very gently sloping hillsides and heavy 
beds of gravel in the bottoms of the valleys, and afterwards that some 
of these valleys were re-excavated and gorges as much as several 
hundred feet in depth were cut down through the old gravels and into 
the bed rock, and beds of gravel were reformed in the bottoms of these 
more gorge-like valleys. 
In other places, where, for various reasons, the effect of the last 
elevation was not felt, the valleys maintained their mature character, 
and in many instances the beds of gravel in the bottoms of these valleys 
were somewhat increased in thickness. 
The gold of the Klondike occurs originally or primarily in quartz 
veins in the chlorite and serecite schist, chiefly of the Klondike Series, 
as defined by Mr. R. G. McConnell. 
These quartz veins are usually lenticular in shape, and rarely con- 
tinue for more than a few feet in horizontal length. As a rule very 
little gold can be seen in them. As a result of a number of assays 
I found that while they usually showed traces of the metal, they 
rarely contained more than $1 to the ton. In some cases, however, 
notably in some quartz veins near the heads of Gay and Victoria gulches, 
two tributaries of Eldorado and Bonanza creeks, these veins were seen 
to contain coarse nuggety gold associated with pyrite. Some of it was 
distinctly crystaline in character, and among the crystals were a number 
of small triangular plates representing “spinel twins,” the twinning 
