SOME GENERAL OBSERVATIONS ON THE WHOLE QUATERNARY. 601 



backs of the veins. 4. Then came the ice-sheet and the glaciers of 

 the Quaternary, like a plow, cutting away the backs of the quartz- 

 veins, together with the containing slates, and, like a mill, grinding all 

 to gravel, and heaping it away in moraines. Some of the placer-mines 

 are in these moraines, but most of the gold has been subjected to still 

 another process. 5. Lastly, in the Champlain epoch, the river-floods 

 washed these moraine heaps down the rivers, sorting them and deposit- 

 ing where the velocity of the current diminished. These river-gravels, 

 thus sorted, cradled, panned by the action of currents, and therefore 

 with the coarse gold near the bottom and high up the gulches, consti- 

 tute the richest placer-mines. 



The placers of California, however, are of two kinds, viz., the ordi- 

 nary or superficial placers, and the deep placers. The superficial placers 

 are gravel-drifts in the present river-beds. The deep placers are gravel- 

 drifts in old river-beds. These old river-beds, as already stated (pp. 

 248, 567), are in many cases covered up with lava. Usually the general 

 direction of the old bed coincides with that of the present river-system, 

 but sometimes the present river-system cuts across the old river-system. 

 In all cases, however, it is evident that the old river-gravels were 

 formed before the lava-flow, and the newer gravels after the lava-flow. 

 In all cases also the present river-system has cut down far below the 

 old beds, in this respect entirely different from the old river-beds of the 

 eastern portion of the continent. The reason of this has already been 

 explained (p. 567). 



The following figures are ideal sections altered a little from Whit- 

 ney's : Fig. 965, of a case in which the old and the present river-beds 

 are parallel to each other ; Fig. 966, where the latter cut through the 

 former. In the former case the section is across the lava-flow, as well 

 as across the river-beds ; in the latter case it is in the direction of the 

 lava-flow, and therefore of the old river-bed, but across the present 

 river-bed. 



In Fig. 965, which is a section across Table Mountain, in Tuolumne 

 County, California, L is the lava-cap, 140 feet thick, beneath which is 



Fig. 965.— Section across Table Mountain. Tuolumne County. California: L, lava; G, gravel; S, 

 slate; _R, old river-bed; £', present river-bed. 



the old river-bed, i?, with its gravel, 6r, now worked by a tunnel, driven 

 through the rim-slate S. More recent gravels, 67', are seen in the pres- 



