CHAP. XLV 
THE ACID BOSSES OF CENTRAL FRANCE 
373 
The evidence for the posteriority of the acid rocks will be fully detailed 
in later pages. Before entering upon its consideration, however, I would 
remark that the uprise of the British granophyres presents so many points 
of resemblance to that of the trachytes and phonolites among the basalt- 
plateaux of Auvergne and the Velay in Central France, that a brief account of 
the acid protrusions of these regions may be suitably given here as an intro- 
duction to the account of those of the Inner Hebrides. A succession of stages 
in the progress of denudation allows us to follow the gradual isolation and 
dissection of the French volcanic groups. The youngest examples occur in the 
chain of cones and craters, in the region of the Buy de Dome. These may 
be of Pleistocene, or even of more recent date. Older and more deeply 
eroded than these are the numerous domes and cones in the territory of 
Haute Loire. Yet more ancient and still more stupendously denuded come 
the bosses, sills and dykes of Britain. Nevertheless, the geologist, by 
the methods so admirably devised by Desmarest, may follow the chain of 
relationship through these different regions and trace a remarkable con- 
tinuity of structure. The younger rocks serve to illustrate the original 
condition of the more ancient, while the latter, by their extensive denudation, 
permit points of structure to be seen which in the former are still concealed. 
No feature in the interesting volcanic district of Auvergne has attracted 
more attention than the trachytic protrusions. 1 . Bising conspicuously along 
the chain of puys, they claim notice even from a distance owing to the topo- 
graphical contrast which their pale rounded domes offer to the truncated, 
crater-bearing cones of dark cinders around them. They consist of masses of 
a pale variety of trachyte (domite), which in ground-plan present a circular 
or somewhat elliptical outline. They vary in size from the nearly circular 
dome of the Grand Sarcoui, which measures about 400 yards in diameter, to 
the largest mass of all — that of the Puy de Dome, which extends for some 
1500 yards from north to south with a breadth varying from 500 to 800 
yards. They are likewise prominent from their height ; in the Puy de 
Dome they form the highest elevation of the whole region (1465 metres), and 
even in the less conspicuous hills they rise from 500 to 600 feet above the 
surrounding plateau. 
Five such dome-shaped protrusions of trachyte have made their appear- 
ance among the cinder-cones in a space of about five English miles in length 
by about two miles in extreme breadth. Though opinions have varied as 
to the mode of formation of these domes, there has been a general agree- 
ment that their present topographic contours cannot be far from the 
original outlines assumed by the masses at the time of their production. 
The position of the trachyte bosses among the puys serves to show that they 
were not deep-seated masses which have been entirely uncovered by denuda- 
1 Tlie admirable Map and Memoirs of Desmarest on Auvergne are classics in geology. Sorope’s 
work, vol. i. p. 45, gives still tlie best published account of this district. See also the work 
Of Lecoq (ibid.). The. results of more detailed petrographical research regarding the rocks 
will be found in the essays of M. Michel Levy (Bull. Sue. Giol. France, 1890, p. 688) and in 
the Clermont sheet of the Geological Survey Map of France (Feuille, 166). A bibliography of the 
district up to the year 1890 is given in the volume of the Bull. Soc. Giol. France just cited, p. 674. 
