Ch.XlX.] 



VOLCANIC ROCKS AUVERGNE. 



259 



The bottom of the hill consists of slightly inclined beds of 

 white and greenish marls, more than three hundred feet in 

 thickness, which are intersected by a dike of basalt, which may 

 be studied in the ravine above the village of Merdogne. The 

 dike here cuts through the marly strata at a considerable angle, 



No. 60. 



Basaltic capping. 

 White and yellow marl. 



Hill of Gergovia. 



producing, in general, great alteration and confusion in them for 

 some distance from the point of contact. Above the white and 

 green marls, a series of beds of limestone and marl, containing 

 fresh- water shells, are seen to alternate with volcanic tuff. In 

 the lowest part of this division, beds of pure marl alternate 

 with compact fissile tuff resembling some of the subaqueous 

 tuffs of Italy and Sicily called peperinos. Occasionally frag- 

 ments of scoriae are visible in this rock. Still higher is seen 

 another group of some thickness, consisting exclusively of tuff, 

 upon which lie other marly strata intermixed with volcanic 

 matter. 



There are many points in Auvergne where igneous rocks 

 have been forced by subsequent injection through clays and 

 marly limestones, in such a manner that the whole has become 

 blended in one confused and brecciated mass, between which 

 and the basalt there is sometimes no very distinct line of de- 

 marcation. In the cavities of such mixed rocks we often find 

 calcedony and crystals of mesotype, stilbite and arragonite. To 



S 3 



