ed ten degrees to the pole beginning
at Gibraltar would pass through a great chain of inland waters--the
Mediterranean, Black Sea, Caspian, Aral, Baikal, and back again through
the great American lakes.

But though many causes have contributed to the original formation and
direction of Valleys, their present condition is mainly due to the
action of water. When we contemplate such a valley, for example, as that
which is called _par excellence_ the "Valais," we can at first hardly
bring ourselves to realise this; but we can trace up valleys, from the
little water-course made by last night's rains up to the greatest
valleys of all.

These considerations, however, do not of course apply to such
depressions as those of the great oceans. These were probably formed
when the surface of the globe began to solidify, and, though with many
modifications, have maintained their main features ever since.

ON THE CONFIGURATION OF VALLEYS

The conditions thus briefly described repeat themselves in river after
river, valley after valley, and it adds, I think, very much to the
interest with which we regard them if, by studying the general causes to
which they are due, we can explain their origin, and thus to some extent
understand the story they have to tell us, and the history they record.

What, then, has that history been? The same valley may be of a very
different character, and due to very different causes, in different
parts of its course. Some valleys are due to folds (see Fig. 41) caused
by subterranean changes, but by far the greater number are, in their
present features, mainly the result of erosion. As soon as any tract of
land rose out of the sea, the rain which fell on the surface would
trickle downwards in a thousand rills, forming pools here and there (see
Fig. 37), and gradually collecting into larger and larger streams.
Wherever the slope was sufficient the water would begin cutting into the
soil and carrying it off to the sea. This action would be the same in
any case, but, of course, would differ in rapidity according to the
hardness of the ground. On the other hand, the character of the valley
would depend greatly on the character of the strata, being narrow where
they were hard and tough; broader, on the contrary, where they were
soft, so that they crumbled readily into the stream, or where they were
easily split by the weather. Gradually the stream would eat into its bed
until it reached a certain slope, the s