He was restrained by two good reasons.

Motoza was likely to seize him before he was swept beyond reach, and if he did not he would inevitably drown.

Accordingly, Fred kept at it until finally they reached the ledge up which Hank Hazletine climbed twenty-four hours later.
By this time a suspicion of the partial truth had penetrated the mind of Fred.

There must be some cavity in the rocks where his captor meant to hold him for awhile as prisoner.

The plan of securing a large ransom in payment for his freedom was not dreamt of by the youth.