[6] The extent of this river erosion has been clearly brought out by Scrope, and is admirably illustrated by several of his panoramic views, such as that in Plate IX. of his work. [7] Scrope, _loc. cit._, p. 147. [8] Scrope, _loc. cit._, p. 144. [9] Scrope gives a view of these remarkable basaltic cliffs in Plate XII. of his work, from which the above account is taken. At one spot near the village of Le Gua there is a break in the continuity of the sheet. [10] See Scrope, _loc. cit._, p. 181; also Appendix, p. 228. While there is no _primâ facie_ reason for questioning the origin of the Demise skull, yet from what Lyell states in his _Antiquity of Man_, p. 196, it will be seen that he found it impossible to identify its position, or to determine beyond question that its interment was due to natural causes. But assuming this to be the case, he shows how the individual to whom it belonged might have been enveloped in volcanic tuff or mud showered down during the final eruption of the volcano of Demise. MM. Hébert and Lartet, on visiting the locality, also failed to find _in situ_ any exact counterpart of the stone now in the museum of Le Puy. [11] See Daubeny, _Volcanoes_, p. 31. [12] That is to say, the surfaces of the lava-streams are not at all, or only slightly, decomposed into soil suitable for the growth of plants, except in rare instances. [13] E. G. Hull, "On the Domite Mountains of Central France," _Scien. Proc. Roy. Dublin Society_, July 1881, p. 145. Dr. Hull determined the density of the domite of the Puy de Dôme to be 2.5, while that of lava is about 3.0. CHAPTER VII. THE VOLCANIC DISTRICT OF THE RHINE VALLEY. The region bordering the Rhine along both its banks above Bonn, and extending thence along the valley of the Moselle and into the Eifel, has been the theatre of active volcanic phenomena down into recent times, but at the present day the volcanoes are dormant or extinct. (_a._) _Geological Structure._--The fundamental rocks of this region belong to the Silurian, Devonian, and Carboniferous systems, consisting of schists, grits, and limestones, with occasional horizontal beds of Miocene sandstone and shale with lignite, resting on the upturned edges of the older rocks. Scattered over the greater part of the district here referred to are a number of conical eminences, often with craters, the bottoms of which are usually sunk much below the present level of the country, and thus receiving the surface drainage, have been converted into little lakes called "maars," differing from ordinary lakes by their circular form and the absence of any _apparent_ outlet for their waters.[1] But before entering into details, it may be desirable to present the reader with a short outline of the physical history of the region (which has been ably done by Dr. Hibbert in his treatise, to which I have already referred), so as to enable him better to understand the succession of physical events in its volcanic history.