380
СHANGE
two in
hemispheres the
systems from
of
one
dence. the
of
hemisphere
the
another,
and
deposits
and
establish
of
in w a t e r s
their
m u d , which
lose
augment
deposits
entirely
narrow
the
streams.
the
somewhat
of
their
channels
that
accumulation
e a r t h , and
points
of
new
the
great hut
branches
connect
bars,
the
confluent at
length
of t h e
rivers
neighbouring
by r a i n - w a t e r s
isthmuses
of
division that did not before
h e n c e results that t h e s e natural c h a n n e l s are by d e g r e e s divided
wherever
their swiftness, con-
of
the
indepen
formed
inundations;
obstruct
different
themselves
complete
are
T h e substances washed down
their
by
abundant
no d o u b t , t o raise t h e b e d s
streams, and
s a m e p l a n e t , w e may c o n c e i v e t h a t
most
running waters
these
RIVER-COURSES.
rivers required m o r e t i m e t o s e p a r a t e
The
tribute,
OF
of
into t w o tributary
form
deposited exist.
It
communication
s t r e a m s , and
from
t h e effect o f a t r a n s v e r s e rising, acquire t w o o p p o s i t e slopes ; a
part
of
recipient,
their w a t e r s is t u r n e d back t o w a r d s and
a
buttress
rises
between
b a s i n s , which o c c a s i o n s all t r a c e s cation to disappear,
from
t h i s period time
the
principal
two
o f t h e i r ancient
l o n g e r c o n n e c t different s y s t e m s o f c o n t i n u e to t a k e place at t h e
the
the
parallel
communi
bifurcations
no
rivers;
and, where they
o f great
i n u n d a t i o n s , we
see that the waters d i v e r g e from t h e principal r e c i p i e n t o n l y to
enter
it
again
after
a longer
or
shorter circuit.
The
l i m i t s , which at first appeared v a g u e and u n c e r t a i n , b e g i n be f i x e d ; ever
a n d in t h e lapse of a g e s , from the action
is m o v e a b l e on t h e surface o f
t h e w a t e r s , t h e d e p o s i t s , and s e p a r a t e , as
great
lose t h e i r ancient
the
globe,
from
of
that
the sands, the basins of
lakes are s u b d i v i d e d , a n d
to
what of
rivers
as inland
seas
communications.*
T h e c e r t a i n t y a c q u i r e d by g e o g r a p h e r s since the s i x t e e n t h century, mutual
of
the
existence
dependence
America,
have
led
of
of
several
various
them
to
bifurcations,
systems
admit
an
of
and
rivers in
the
South
intimate connection
* The geological constitution of the soil seems to indicate that, not withstanding the actual difference of level in their waters, the Black Sea, the Caspian, and lake Aral, communicated with each other in an era anterior to historic times. The overflowing of the Aral into the Caspian Seaseemseven to be partly of a more recent date, and independent of the bifurcation of the Gihon (Oxus), on which one of the most learned geo graphers of our day, M . Ritter, has thrown new light.