THE RHINE. 
197 
Volcanic District of Hohgau. 
Volcanic phenomena play a very subordinate part 
in the physical geography of Switzerland. No doubt 
the masses of Granite, Porphyry, Syenite, Gabbro, 
Diorite, etc., sufficiently indicate the existence of 
plutonic forces, but the enormous denudation which 
has taken place has long, ago removed all the surface 
rocks, leaving only “necks” or volcanic chimneys like 
those found in Scotland and elsewhere. 
Fig. 137* Volcanic group of tho liohguu. Seen from the north-west. 
The district of Hohgau, north-east of Schaff- 
hausen, is however an exception. Here there is a 
group of comparatively modern, but extinct, vol- 
canoes (Fig. 1 37). Some are basaltic — Reidheim, 
Hohenstoffein, Hohenhowen; others are phonolitic — 
Hohentwiel, Magdeberg, and Hohenkrahen. They 
appear to have been of Miocene age, as phonolitic 
tuffs, probably derived from them, are interstratified 
with the freshwater deposits of Oeningen. 
The hills rise to a height of 850 metres, steeply 
