Questions 13-16 are based on the following passage.

    To account for the conformation of the Alps, two hypotheses have been advanced, which may be respectively named the hypothesis of fracture and the hypothesis of erosion. The former assumes that the forces by which the mountains were elevated produced fissures in the earth’s crust and that the valleys of the Alps are the tracks of these fissures; the latter maintains that the valleys have been cut out by the action of ice and water, the mountains themselves being the residual forms of this grand sculpture. | had heard the Via Mala cited as a conspicuous illustration of the fissure theory—the profound chasm thus named, and through which the Hinter-Rhein now flows, could, it was alleged, be nothing else than a crack in the earth’s crust. To the Via Mala | therefore went in 1864 to instruct myself upon the point in question.

    The gorge commences about a quarter of an hour above Tusis and, on entering it, the first impression certainly is that it must be a fissure. This conclusion in my case was modified as | advanced. Some distance up the gorge | found upon the slopes to my right quantities of rolled stones, evidently rounded by water-action. Still further up, and just before reaching the first bridge which spans the chasm, | found more rolled stones, associated with sand and gravel. Through this mass of detritus, fortunately, a vertical cutting had been made, which exhibited a section showing perfect stratification. There was no agency in the place to roll these stones, and to deposit these alternating layers of sand and pebbles, but the river which now rushes some hundreds of feet below them. At one period of the Via Mala’s history the river must have run at this high level. Other evidences of water-action soon revealed themselves. From the parapet of the first bridge | could see the solid rock 200 feet above the bed of the river scooped and eroded.


With specific reference to the author's conclusion based on the observation of rolled stones and pebbles in the gorge, which of the following is an assumption that the author relies on?