[Illustration: Fig. 18.--Diagram illustrating the structure and denudation of Puys.] The accompanying diagram (Fig. 18) may help to make these changes more intelligible. The upper dotted lines show the original forms of three puys with the covering of loose materials discharged by them over the surrounding ground. The lower shaded portion represents the surface as left by denudation, and a section of the three vents beneath that surface. The whole of the cones and craters has here been swept away, and only the volcanic "neck" is in each case left. In the vent to the right, the material that fills it up is a coarse agglomerate, which projects as a rounded dome above the surrounding country. The central pipe is filled with fragmentary materials, through which molten rock has risen, giving off dykes and veins. In the vent to the left hand, only lava is seen to occupy the orifice, representing the column of molten rock which solidified there and brought the activity of this little volcano to an end. It will be observed that in each of these volcanic hills the present outlines are very far from being those of the original volcano, and that the eminence projects because of its greater resistance to the forces of denudation that have not only removed the superficial volcanic material, but have made some progress in lowering the level of the ground on which that material was accumulated. The typical area for the study of Puys is the extraordinarily interesting volcanic region of Central France. There the volcanic cones are clustered in irregular groups, sometimes so close as to be touching each other; elsewhere separated by intervals of several miles. They may be traced in all stages of decay, from the most perfect cones and craters to the isolated eminence that marks the site of a once active chimney. Their lavas, too, may be seen as detached fragments of plateaux, many hundred feet above the valleys that have been excavated since they flowed.[22] [Footnote 22: See Desmarest's classic map and his papers in _Mem. Acad. Roy. Sciences_, Paris, 1774, 1779; _Journ. de Physique_, 1779; Scrope's _Geology of Central France_, 1827, and _Extinct Volcanoes of Central France_, 1858; Lecoq's _Époques Géologiques de l'Auvergne_, 1867; M. Michel Lévy, _Bull. Soc. Géol. France_, 1890, p. 688; M. Boule, _Bull. Carte Géol. France_, No. 28, tome iv. 1892.]