CHAP. XLV 
THE ACID BOSSES OF CENTRAL FRANCE 
37 5 
paratively acid trachytes was not the last volcanic episode of the district. 
The abundance of dark slags and fragments of basalt lying on the domite hills 
shows that discharges of more basic detritus occurred after these hills had 
taken their place in the landscape. 
Since the latest eruptions, a gradual alteration of the topographical features 
by denudation has been slowly but continuously going on. The Grand 
Sarcoui, possibly from having originally had a considerable covering of frag- 
mentary material, shows least the effects of this waste. Its remarkably 
regular form, like that of an inverted cauldron (the “ Chaudron,” as it is 
called in the district), presents, in a distant view, a smooth grassy surface 
which slopes steeply down into the great volcanic plain. But on a nearer 
examination these declivities are found to he seamed with trenches which 
the rain-storms of centuries have dug out. The covering of loose debris has 
been largely washed away, though many fragments of dark slag are still 
strewn over the slopes, and the scars are now being cut into the domite 
below. A more advanced stage of decay may be seen on the Puy de Dome, 
where, from greater elevation and exposure, the domite is already deeply 
gashed by gullies and ravines, while the slopes below are strewn with its 
detritus. 
The region of the Yelay displays on a far more extensive scale the pro- 
trusion of traehytic and phonolitie bosses, but as its volcanic history goes 
back beyond the time of the Puys of Auvergne, its volcanic monuments 
have consequently been more extensively affected by denudation. 1 A series 
of basaltic eruptions forming extensive sheets can there be traced, the oldest 
dating from Miocene time, the youngest coming down to the age of the 
mammoth, cave-bear and early man. During this prolonged outpouring 
of basic lavas there were several intervals during which materials of a more 
acid nature — trachytes and phonolites — were erupted. These rocks occur 
partly as extensive tracts, covering five or six square miles, like those of the 
Mezenc, the Megal, the Pic de Lizieux, and the Band, and partly in isolated 
conical or dome-shaped prominences, sometimes only a few hundred feet in 
diameter. Upwards of one hundred distinct eruptions of plionolite have 
been observed in the Yelay. Even in the tracts where they cover the 
largest space, several prominent eminences may usually be observed, not 
unlike in general shape tire isolated cones and domes of Auvergne. In 
these wider areas there appears to be evidence of the outcome of the lava 
from one or more vents, either as superficial streams or as underground 
intrusive sheets. M. Boule lias expressed his opinion that most of the 
masses of trachyte and phonolite have been the result of local and limited 
eruptions, the pasty rock having risen in and accumulated around its pipe, 
without flowing far in any direction. A section across one of these masses 
would present a somewhat mushroom-shaped form." 
1 In addition to the work of Scrope, the student of this important volcanic district will find 
an invaluable guide in the Le Puy Sheet (No. 186) of the Geological Survey Map of France, and 
in the. Bulletins of the Survey, particularly those by MM. Termier and Boule, No. 13 (1890) and 
No. 28 (1892). 
2 Bull. Carte. Geol. France, No. 28 (tome iv.) p. 125. 
