Aboriginal Races of America [pp. 59-92]

1853.] Aboriginal Races of America. 81
fact that none of the ancient monuments occur upon the latest formed
terraces of the river valleys of Ohio, is one of much importance in its
bearing upon this question. If, as we are amply warranted in believ ing, these terraces mark the degrees of subsidence of the streams, one of
the four which may be traced, has been formed since those streams have
followed their present courses. There is no good reason for supposing
that the mound builders would have avoided building upon that terrace,
while they erected their works promiscuously upon all the others.
And if they had built upon it, some slight traces of their works would
yet be visible, however much influence one may assign to disturbing
causes- overflowss, and shifting channels. Assuming then, that the
lowest terrace, on the Scioto River, for example, has been formed since
the era of the mounds, we must next consider that the excavating pow er of the Western rivers diminishes yearly, in proportion as they ap proximate towards a general level. On the lower Mississippi-where,
alone, the ancient monuments are sometimes invaded by the water the bed of the stream is rising, from the deposition of the materials
brought down from the upper tributaries, where the excavating process
is going on. This excavating power, it is calculated, is in an inverse
ratio to the square of the depth, that is to say, diminishes as the square
of the depth increases. Taken to be approximatively correct, this rule
establishes, that the formation of the latest terrace, by the operation of
the same causes, must have occupied much more time than the forma tion of any of the preceding three. Upon these premises, the time,
since the streams have flowed in their present courses, may be divided
into four periods of different lengths-of which the latest, supposed
to have elapsed since the race of the mounds flourished, is much the
longest."
"The fact that the rivers, in shifting their channels, have, in some
instances, encroached upon the superior terraces, so as, in part, to destroy works situated upon them, and afterwards rended to long distances of a fourth, or half a mile, or upwards, is one which should not
be overlooked in this connection. (See pages 50, 60, and 89.) In the
case of the'high bankworks,' (plate xvi.) the recession has been nearly
three-fourtlis of a mile, and the intervening terrace, or'bottom,' was,
at the period of the early settlement, covered with a dense forest. This
recession, and subsequent forest growth, must, of necessity, have taken
place since the river encroached upon the ancient works here alluded to.
"Without doing more than to allude to the circumstance of the exceedingly decayed state of the skeletons found in the mounds, (see page
168,) and to the amount of vegetable accumulations in the ancient ex 6