THE GEOLOGICAL AGE OF THE GRAVEL 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- 
sentatives 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 does 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 were undoubtedly high up in the range, in the 
gravel 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 agencics 
ceased their activity, at least over all the mining region of the Sierra, cannot 
be doubted. The fact is demonstrated by the large 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 
