They occur at heights varying from 10 to 100
feet above the present river-channels, and they are therefore
older than the recent gravels by the time required by the river
to dig out its own bed to this depth. How long this period may
be, our data do not enable us to determine accurately; but if
we are to calculate from the observed rate of erosion of the
actually existing rivers, the period between the different
valley-gravels must be a very long one.
The lowest or recent fluviatile deposits which occur beside the
bed of the present river, are referable to the Recent period,
as they contain the remains of none but living Mammals. The two
other sets of gravels are Post-Pliocene, as they contain the
bones of extinct Mammals, mixed with land and fresh-water shells
of existing species. Among the more important extinct Mammals
of the low-level and high-level valley-gravels may be mentioned
the _Elephas antiquus_, the Mammoth (_Elephas primigenius_),
the Woolly Rhinoceros (_R. Tichorhinus_), the Hippopotamus, the
Cave-lion, and the Cave-bear. Along with these are found
unquestionable traces of the existence of Man, in the form of
rude flint implements of undoubted human workmanship.
The so-called "Cave-deposits," again, though exhibiting peculiarities
due to the fact of their occurrence in caverns or fissures in the
rocks, are in many respects essentially similar to the older
valley-gravels.
Pages:
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508