There cannot exist a doubt that the district in question was the seat of an extensive chain of lakes, imbosomed amidst the primary rocks, and silted up during the currency of the tertiary age, partly by sedimentary and partly by igneous matter. The unstratified masses which encircled their waters, still stand out in bold relief from the well-defined strata that now occupy their basins. A walk up any one of these valleys—and they are innumerable—or among the cones, hundreds of which are scattered over the high grounds in the vicinity of Gergovia, will present to you in striking contrast these extremes of natural masonry. One can almost trace, in some localities, the very fissures which opened in the sides of the granite rocks, whence issued the molten flood that first perturbed the waters of the pure silent lakes. No straining of the imagination is indeed required to trace the whole progress of their silting—now in the dark lava-current from the bowels of the earth, and now in the collected debris from the mountain sides, hurried down by the torrent or by their own convulsive throes—here the fine comminuted sand, gently carried in by the stream, and there the waste of animal life forming entire beds of calcareous marls of still unsullied freshness. In the whole range of geology there is not, in fact, to be found anything more instructive and interesting than is displayed in these lacustrine deposits, the extreme thinness often of the beds, and the beautiful regularity of their superposition. The lavas intermix, and alternate repeatedly, with the alluvial and organic strata. A myriad of trickling rills fling themselves from the upheaved ridges, so green and flowery to their summits; they are collected into streams in the different ravines, and sweeping through the deep-cut gorges, lay open the interior to the depth of many hundred feet. Here the various igneous and aqueous groups can be read and studied in detail, as they were quietly deposited or violently strewn upon one another.