CHAP. IV 
PUY TYPE 
45 
and ashes. Hence the products of such a vent group themselves into three 
layers— two of breccia separated by an intervening sheet of lava.^ 
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 he effaced, and the lava 
itself may he reduced to merely a few separate patches, perhaps capping a 
rid"e, while the surrounding ground has been hollowed into valleys. It the 
waste continues long enough, even these outliers will disappear, and nothing 
but the neck or stump of the little volcano will remain. 
H’he accompanying diagram (Tig. lH) 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 sur- 
roundino- 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 super- 
ficial volcanic material, but have made some progress in lowering the level of 
the ground on which that material was accumulated. 
The tyyjical area for tlie 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 ; else- 
where 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 e.xcavated since they flowed." 
1 M. Boiile, Bull. CaHc Giul. France, No. 28, tome iv. 
^ Sec DeHiaarest’s classic map anil his pajiers in Mcm. Acad. 
Jouni. dc Physiqiic^ l77^^ ; Hcrope’s (k.olugy of VeMtral l^rancc. 
Jloil. Sciences^ Paris, 1774, 1/79 ; 
, 1827, and E-ctinct Volcanoes of 
