VIII NATURAL ARCHES--THEIR SIGNIFICANCE AND THEIR INFLUENCE Long before the germ of humanity in some anthropomorphous apes became slowly fertile in a mysterious gestation, Nature had weathered many rocks into hollowed and vaulted shapes. Some were yawning sea-caves, whose arched mouths gulped in the tidal waves, and whose caverned bodies gurgled or boomed with the noise of deepening water.[55] Others were vaults gradually fretted into being by subterranean torrents, such as we find to-day at Saint-Pons, in the Cevennes, where the river Jaur is nourished by an abundant spring which in a second, through the mouth of a low-arched cavern, pours a thousand litres of fresh, sweet water. Others, again, were genuine arched bridges, such as we find to-day in the Pont d’Arc, over the river Ardèche (p. 6). In England we have several such bridges, notably the Durdle Door on the coast at Lulworth, whose arched span must owe at least a part of its shape to the troubled action of sea-waves. “La Roche Percée” at Biarritz--a crinkled, lava-like formation--is inferior to our Durdle Door; and “La Roche Trouée,” near Saint-Gilles Croix-de-Vie, though remarkable as a square-headed aperture, has a lower place still in the pontine work done by Nature.[56] Perhaps the most wonderful rock-bridges are those at Icononzo, in New Grenada, over the torrent of Summa-Paz. There are two, and one of them soars up and up to a crown that spans the water at an altitude of ninety-seven metres. How could men of genius fail to be architects when Nature set before their eyes great vaults, not only varied in shape, but at times of a stupendous height? In different ways she produced surbased arches, pointed arches, semicircular arches, all more or less ragged in their outlines, but each a model for progressive mimicry and adaptation. Here is not the place to dally with the causes of their formation, such as uneven weathering and the scour of running water subject to high tides or to terrific floods. As rivers in the course of many ages deepen and widen their channels, they reach now and then a strata of fissured rock, and their eating action is very rapid when they are able to undercut the softer rocks by fretting their way along apertures or crevices. Many an earthquake has made such inlets for river water, and earthquakes may have shattered some rocks into vaulted shapes. Whether glaciers have played a part in the hollowing of rocks into arched caves and bridges I do not know; but rock-basins are attributed to the erosive power of glaciers, so why not some rock-bridges also? It is a question over which geologists ought to quarrel as they did over rock-basins.[57] [Illustration: PONTE DELLA PAGLIA AT VENICE, _RENAISSANCE_]