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140 the surfaces which are not opposed to the general direction of the current. In all cases concavities are being produced. Projecting angles are rounded, it is true, and are more or less convex, but they are overlooked on account of the prevalence of concave forms.

Cause and effect help each other here. The inequalities of the torrent bed and walls cause its eddyings, and the eddies fashion the concavities. The more profound the latter become, the more disturbance is caused in the water. The destruction of the rocks proceeds at an ever-increasing rate; for the larger the amount of surface that is exposed, the greater are the opportunities for the assaults of heat and cold.

When water is in the form of glacier it has not the power of making concavities, such as these, in rocks, and of working upon surfaces which are not opposed to the direction of the current. Its