If we consider that in basalt-flows the lower part is compact, and often divided into prismatic columns, while the upper part is porous, cellular, scoriaceous, and irregularly divided--that the points of separation on which they rest are small beds presenting fragments of the porous stony concretions known under the name of _Lapilli_--that the lower portions of these masses present a multitude of points which penetrate the rocks on which they repose, thereby denoting that some fluid matter had moulded itself into its crevices--that the neighbouring rocks are often calcined to a considerable thickness, and the included vegetable remains carbonised--no doubt can exist as to the igneous origin of basaltic rocks. When it reached the surface through certain openings, the fluid basalt spread itself, flowing, as it were, over the horizontal surface of the ground; for if it had flowed upon inclined surfaces it could not have preserved the uniform surface and constant thickness which it generally exhibits. [Illustration: III.--Extinct volcanoes forming the Puy-de-Dôme Chain.] VOLCANIC OR LAVA FORMATIONS. The _lava_ formations comprehend both extinct and active volcanoes. “The term,” says Lyell, “has a somewhat vague signification, having been applied to all melted matter observed to flow in streams from volcanic vents. When this matter consolidates in the open air, the upper part is usually scoriaceous, and the mass becomes more and more stony as we descend, or in proportion as it has consolidated more slowly and under greater pressure.”[21] [21] “Elements of Geology,” p. 596. The formation of extinct volcanoes is represented in France by the volcanoes situated in the ancient provinces of Auvergne, Velay, and the Vivarais, but principally by nearly seventy volcanic cones of various sizes and of the height of from 500 to 1,000 feet, composed of loose scoriæ, lava, and pozzuolana, arranged upon a granitic table-land, about twelve miles wide, which overlooks the town of Clermont-Ferrand, and which seem to have been produced along a longitudinal fracture in the earth’s crust, running in a direction from north to south. It is a range of volcanic hills, the “chain of _Puys_” nearly twenty miles in length, by two in breadth. By its cellular and porous structure, which is also granular and crystalline, the felspathic or pyroxenic lava which flowed from these volcanoes is readily distinguishable from the analogous lavas which belong to the basaltic or trachytic formations. Their surface is irregular, and bristles with asperities, formed by heaped-up angular blocks.