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to . tbrra. pretty accurate computations oftheir distance from the sea, by observingthe courses of rivers : if a river runs near-ly straight, for fifteen or twenty leagues,they know themselves to be a great wayfrom the coast: if there are many sinu-osities, they conclude that they are not farfrom the sea.

■ Valleys have a tendency to increase gra-dually in breadth as they descend, modifiedhowever by two disturbing causes, which Ishall presently have occasion to notice. Anecessary consequence of this tendency isthe gradual tapering of promontories. Thewedgelike form, so strikingly exemplified inthe peninsulas of Africa and South America ,is found for the most part in individual hillsand mountains, the broad end fronting thehead a of the valley.

Now in regard to disturbing causes :—

The breadth of valleys depends insome measure on the comparative hardnessof the substances which bound them. The

. - * Transactions R. S, Edin. voL vii. p. 170.

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