THE GEOLOGICAL AGE OF THE GEAVEL DEPOSITS. 



241 



to Dr. Leidy's work, will be to bring this scattered material together, accom- 

 panying it with such purely geological comments as shall seem required for 

 enabling the reader to get the most light possible on the question of the 

 geological age of the formations rendered so interesting not only by their 

 richness in gold, but by the presence in them of human remains. 



From what has been said in the preceding chapters, it will be apparent 

 that one of the first and most difficult questions to be settled in the discus- 

 sion of the age of the auriferous gravel series will be, Where is the line to 

 be drawn between the epoch of the present and that of the past ? The 

 whole body of material with which we have to do is, geologically speaking, 

 of very recent age. There is evidently a passing, as we trace the progress 

 of events backwards, from circumstances and conditions actually existing 

 to quite different ones. Yet the striking analogy between former conditions 

 and those now prevailing cannot but have been impressed on the mind of 

 the reader of this volume. The gravels were then as now the result of 

 fluvatile action: the rivers which did the work of rounding and polishing 

 the innumerable boulders and pebbles which those older deposits contain 

 are doing the same thing now, although with diminished power. The very 

 channels in which those former currents ran have, in most cases, their repre- 



t 



sentativcs now; at a lower level, and on a diminished scale, it is true, but 

 still essentially the same, since modern and ancient streams do not prob- 

 ably differ very much in their areas of drainage. 



What docs essentially distinguish the present epoch from a former one is 

 the cessation of volcanic energy. For it has been abundantly shown in the 

 preceding chapters that, during the formation and deposition of a portion of 

 the auriferous detritus, the gravel region was the scene of powerful and per- 

 sistent eruptive action, the seat of which extended through the whole of the 

 Sierra. The volcanic vents w T ere undoubtedly high up in the range, in the 



ravel region at least, as will be evident to any one examining the large 

 map appended to this volume, as well as from the description of the position 

 of the various detrital masses which has been given in the preceding pages. 

 This peculiar geological phase of events has now entirely passed by, and 

 that a very considerable period of time has elapsed since volcanic agencies 

 ceased their activity, at least over all the mining region of the Sierra, cannot 

 he doubted. The fact is demonstrated by the lar^e amount of erosion which 



the most recently erupted masses have undergone, and which has been re- 

 peatedly mentioned and commented on while describing the gravels and 



