d to a spot a mile and a half distant from that where it
accumulated in the tenth century, and six miles from the point where the
delta began originally to form. We may look forward to the period when
this lake will be filled up, and then the distribution of the
transported matter will be suddenly altered, for the mud and sand
brought down from the Alps will thenceforth, instead of being deposited
near Geneva, be carried nearly 200 miles southwards, where the Rhone
enters the Mediterranean.

In the deltas of large rivers, such as those of the Ganges and Indus,
the mud is first carried down for many centuries through one arm, and on
this being stopped up it is discharged by another, and may then enter
the sea at a point 50 or 100 miles distant from its first receptacle.
The direction of marine currents is also liable to be changed by various
accidents, as by the heaping up of new sand-banks, or the wearing away
of cliffs and promontories.

But, secondly, all these causes of fluctuation in the sedimentary areas
are entirely subordinate to those great upward or downward movements of
land which have been already described as prevailing over large tracts
of the globe. By such elevation or subsidence certain spaces are
gradually submerged, or made gradually to emerge:--in the one case
sedimentary deposition may be suddenly renewed after having been
suspended for ages, in the other as suddenly made to cease after having
continued for an indefinite period.

_Causes of variation in mineral character of successive sedimentary
groups._--If deposition be renewed after a long interval, the new strata
will usually differ greatly from the sedimentary rocks previously formed
in the same place, and especially if the older rocks have suffered
derangement, which implies a change in the physical geography of the
district since the previous conveyance of sediment to the same spot. It
may happen, however, that, even when the inferior group is horizontal
and conformable to the upper strata, these last may still differ
entirely in mineral character, because since the origin of the older
formation the geography of some distant country has been altered. In
that country rocks before concealed may have become exposed by
denudation; volcanoes may have burst out and covered the surface with
scoriae and lava, or new lakes may have been formed by subsidence; and
other fluctuations may have occurred, by which the materials brought
down from thence b