376 
THE TERTIARY VOLCANOES 
BOOK VIII 
That fragmentary ejections accompanied the protrusion of these rocks, 
though probably on a very limited scale, is shown by the occasional survival 
of portions of trachyte tuff around them. One of the most notable of these 
deposits occurs in the hollow between the Sue du Pertuis and the next dome 
to the south. It consists of fine and coarse, trachytic detritus, which in 
one place is rudely bedded and appears to dip away from the phonolite 
dome behind it at an angle of 30°. This material and its inclination are 
what might be expected to occur round an eruptive vent, and may be com- 
pared with those of the crater- wall of the Puy de la Goutte in relation to 
the domite boss of the Puy de Chopine. 
The denudation of Velay has undoubtedly advanced considerably further 
than that of the Puys of Auvergne. The pyroclastic material which may 
have originally covered the domes of trachyte and phonolite has been in 
great part swept away. The surrounding rocks, too, both aqueous and 
igneous, have been extensively removed from around the necks of more 
enduring material. Hence the trachyte and phonolite bosses stand out with so 
striking a prominence as to arrest the eye even for a distance of many miles. 
There cannot be any doubt that these necks have pierced the older 
Fig. 345. — View of the Huche Pointue and Huehe Platte west of Le Pertuis. 
The cone is one of the trachytic domes, while the flat plateau to the left is a denuded outlier of the basalt sheets. 
basalts, and therefore belong to a later epoch in the volcanic history. The 
approximately horizontal sheets of basalt have been deeply eroded and re- 
duced to mere fragments, and in some instances their existing portions owe 
their survival to the protection afforded to them by the immense protrusions 
of more acid material. But there is here, as well as in Auvergne, evidence 
of the uprise of a later more basic magma, for sheets of basalt are found 
overlying some parts of the trachytes and phonolites. 
While the external forms of these Velay necks recall with singular 
vividness the features of many more ancient necks in Britain, an examina- 
tion of the internal structure of some of them affords some further interest- 
ing points of resemblance. The slabs into which, by means of weathering 
along the joints, the rock is apt to split up are sometimes arranged with a 
general dip outwards from the centre of the hill, so that their flat surfaces 
roughly coincide with the hill-slopes. In other cases the peculiar platy struc- 
ture, so characteristic of phonolite, is disposed vertically or dips at a steep 
angle into the hill, so that the edges of the slabs are presented to the 
declivities, which consequently become more abrupt and rugged. 
