, 1877-08-04
sediments.

the world can be had reckoning from the
present time back for great many cen
turtles, than the sedimcntary deposits OF
streams. The annual increase OF these
deposits has been regularly measured
for great length Of time, the average
deposit per amman noted, and from that
data calculating the number Of years
that must eapsc in order to produce II
deposit of certain depths. This npcthod
only proximates accuracy, as the depths
OF deposits vary each year, owing to
the effect that variation of temperature
has upon the volume Of water and its ac.
companying sediment that given streams
carry to the bays or oceans into which
they empty. Civilization tends also to
increase the amount of sedimcntury de.
| posts in nearly all streams. Navigation
also disturbs the deposits already made
rendering the water turbid, and causing
an unnatural quantity to be carried to
greater ocean depths, Swallowing the
water in some places and increasing its
depths in Others
For these and many other reasons
measurements OF sedimentary accumula.
tions at the mouths OF Streams, in bays,
harhors and along ocean beaches cannot
be depended upon, as some have suppos-
ed, AS correct indices Of the lapse of
time.
Such II stream, however, rs the Nile,
which annually Cverilows, and deposits
its sediment on its banks, and which is
known to have missed its regular annual
overflow only twice in nearly 1000 years,
affords Il data accurate and reliable, be-
cause there is nothing to disturb these
deposits after they are once made.
Excavations at various points have
revealed the fact that 1t would require
over 10000 years for that stream to de
post its present depth of soil. Streams
that overflow each year regularly, and
only once, leaves Il layer each year; and
by counting the successive layers, you
arrive at the time that has elapsed since
these deposits commenced.
But in this case even the estimate
cannot be accurately made, because,
after excavating toa certain depth, the
great pressure from above has obIiteated
the layer marks. And, even if this was
not the case, at certain depth, point
would be reached where the deposits
were made at II time when the point ex-
amined was an ocean bed, subject to the
same causes as disturbs ocean and harbor
deposits at the present time, consequent
ly inaccurate.
These examinations, then, are valuable
only as forming reliable data as. to the
time that has elapsed since the valley has
been filed up sufficiently to confne the
stream to its present channel, except at
the period Of its annual overflow These
figures have given the astonishing num-
Der Of over 10000 layers. However, his
is really only a short period, when con-
trasted with the time required for the
zoophytes to build up coral reefs from
the bed OF the ocean.
Another interesting fact in connection
with the deposits of annual overflow is,
that they must have all occurred in the
modern period, as no traces of geological
change have been found among the de-
bris Of the layers. And when we remem-
Der that we can number the overtlows
Of the Nile only after the sediment car
ried by the stream had filed up the
valley, actually redeemed it from the sea
and removing its present outlet num-
Der Of leagues inland, we have reliable
data only for the htest portion of the
modern geological period.
We should be completely at fault, and
could form no accurate data at all, for
any PERIOD were it not for other facts
that assist us in solving such problems.
When sedimentary deposits, on account
of their depth and the pressure from
above, begin to he chaotic, and of unre-
liable occurrence, s;ratitied rocks take up
the calculation, and carry us back such
All immense period OF time, that we begin