GEOLOG Y 51
open valleys, with rivers near their base-level of erosion, and
the gently undulating plains are due to the toning down of the
rocks by long exposure to the weather.
A very different state of things is disclosed in the land lying The folded
to the west, north, and east of the great Indo-Gangetic alluvial extra-
belt: in Sind, Baluchistan, the Himalayan belt, Assam, and area.
Burma we have abundant evidence of repeated immersions
beneath the ocean. In this area the directions of the mountain
chains are determined by comparatively young rock-folds, while
the region having been but lately elevated, its rivers are swift
and torrential, cutting down their beds so rapidly that the
valley sides are steep, with loosened material, always ready to
slide off in destructive landslips.
In attempting to express these two distinct geological stories Correla-
in European terminology we find that our simplest and most tion of
easily translated characters are preserved in the marine fossili- strata with
ferous strata. while it is practically impossible to correlate the
directly the land and fresh-water formations which are so largely scale.
developed on the Peninsula with their equivalent stages in the
European standard scale.
The reasons for this contrast are simple. Conditions of life Variable
are much more uniform, and facilities for migration much evolution
more perfect, in the ocean than on land. On land areas there in isolated
is a greater variety of physical features, and a greater diversity, land areas.
therefore, of climate and other conditions which affect the
distribution of living beings. Consequently, when such areas
are cut off from one another by impassable physical barriers,
the intermingling of plants and animals is prevented, and
evolution proceeds at independent rates in the separated areas,
attaining corresponding stages at quite different times. As an
example of the errors which would arise if we compared the
fresh-water and land fossils of widely separate areas with one
another, we have, in the existing indigenous mammalian fauna
of the isolated Australian continent, a stage of evolution
about equivalent to that which characterized Europe in Jurassic
times. This want of correspondence during the same period
of living forms in widely-separated land areas is one of great
importance to the Indian geologist, who has had the point
most strikingly brought home to him in his attempt to deter-
mine the age of the great coal-bearing system in India. The
luxuriant growths of ferns, horsetails, cycads, and conifers
which flourished in the great river-valleys of the old Gondwana
continent did not make their appearance in Europe until well
on in the Mesozoic era, yet, from other evidence, we know