[20] M. Boule, Bull. Carte Géol. France, No. 28, tome iv. p. 232.
In the history of a puy there is commonly a first discharge of fragmentary material; then lava may flow out, followed by a final discharge of loose stones and ashes. Hence the products of such a vent group themselves into three layers—two of breccia separated by an intervening sheet of lava.[21]
[21] M. Boule, Bull. Carte Géol. France, No. 28, tome iv.
Great changes are wrought on puys and their connected lavas and tuffs during the progress of denudation. The cones are eventually destroyed, and only a stump of agglomerate or lava is left to mark its place. The connection of a lava-stream with its parent vent may likewise be effaced, and the lava itself may be reduced to merely a few separate patches, perhaps capping a ridge, while the surrounding ground has been hollowed into valleys. If the waste continues long enough, even these outliers will disappear, and nothing but the neck or stump of the little volcano will remain.
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]
[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.
Another well-known region of modern Puys is that of the Eifel, where the cones and craters are often so fresh that it is difficult to believe them to be prehistoric.[23] One of the most remarkable denuded puy-regions in Europe covers a wide territory in the Swabian Alps of Würtemberg. No fewer than 125 denuded necks filled with tuff, agglomerate and basalt have there been mapped and described. They are of higher antiquity than the Upper Miocene strata, and have thus probably been exposed to prolonged denudation. In external aspect and internal structure they present the closest parallel to the Carboniferous and Permian "necks" of Britain described in Books VI. and VII. of the present work.[24]