CHAP. XLV 
THE ACID BOSSES OF CENTRAL FRANCE 
377 
Though none of the volcanic series in Auvergne or the Velay is so acid 
in composition as the more acid members of the Tertiary volcanic ' series of 
Britain, the manner in which the trachytes and phonolites of the french 
region make their appearance presents some suggestive analogies to that of 
the corresponding rocks in tins country. We see that they were erupted long- 
after the outpouring of extensive basaltic plateaux, that they belonged to 
successive epochs of volcanic activity, that they were protruded in a pasty 
condition to the surface, where, more or less covered with fragmentary 
ejections, they terminated in dome-shaped hills or spread out to a limited 
distance around the vents, and lastly, that they were succeeded by a still 
later series of more basic eruptions, which completed the long volcanic 
history. We shall see in the following pages how closely the various stages 
in this complex record of volcanic activity may be paralleled in the geo- 
logical records of Tertiary time in Britain . 1 
1 The phonolite necks of Bohemia, which form so prominent a feature in the Tertiary geology 
of that country, might likewise be cjted here in illustration of the acid domes and bosses of the 
British Isles. 
