The thermal waters of Algeria are, however, surpassed in grandeur and beauty by the springs of the ancient Ionian city of Hierapolis (holy city), which at the present time flow in the solitary plateau called Panbouk-Kelessi (Castle of Cotton), on account of the cotton-like aspect of the white masses of travertin of which it is composed. On reaching this spot from Smyrna, something like an immense cataract may be seen in the distance, 328 feet high and 2½ miles wide; this is formed by the walls which the water has gradually constructed, column after column, and layer after layer, by flowing over the edges of the plateau and gushing out on the slopes. Here and there, real cascades glitter in the sun, and their sparkling surfaces light up the dead whiteness of the crystal walls. As a spectator ascends the declivities, the masses deposited and carved out by the water appear in all their strange beauty; one might fancy that they were colonnades, groups of figures, and rude bas-reliefs which the chisel had not yet perfectly set free from their rough coverings of stone. And all these calcareous deposits which have been fashioned by the cascades during a succession of ages open a multitude of cup-like hollows with fluted edges fringed with stalactites; these graceful reservoirs--some of which are shaded with yellow or veined with red, brown, and violet, like jasper or agate--are filled with pure water. Higher still follow two steps of the plateau on which stood the ancient thermal edifice and the Necropolis of Hierapolis. There whitish masses cover the ancient tombstones and fill up the conduits. The ground is crossed in various directions by the former beds of rivulets, which have gradually stopped up their own courses by depositing concretions upon them. Above one of the widest of these dried-up channels, the magnificent span of a natural bridge displays its graceful form, like an arch of alabaster, streaming with innumerable stalactites. At what date did this majestic structure take its rise, and how many years and centuries did the process of its formation last? No one knows. According to Strabo, the channels of the baths of Hierapolis were soon filled up by solid masses, and if Vitruvius can be believed, when the proprietors of the environs wished to inclose their domain, they caused a current of water to run along the boundary-line, and in the space of a year the walls had risen. Silica, which is still more important than chalk in the formation of terrestrial rocks, is also sometimes deposited on the edge of springs, but in very small quantities.