554 



EOCENE VOLCANIC ROCKS. 



[Oh. XXXn. 



alternation here of a contemporaneous sheet of lava with freshwater 

 strata in the manner supposed by some other observers ;* but the posi- 

 tion and contents of some of the associated tuffs, prove them to have 

 been derived from volcanic eruptions which occurred during the deposi- 

 tion of the lacustrine strata. 



The bottom of the hill consists of slightly inclined beds of white and 

 greenish marls, more than 300 feet in thickness, intersected by a dike 

 of basalt, which may be studied in the ravine above the village of Mer- 

 dogne. The dike here cuts through the marly strata at a considerable 

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

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



Fie. 6TS. 



White 

 and green 

 marls. 



Hill of Gergovia. 



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. Oc- 

 casionally fragments 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. 

 Among the species of fossil shells which I found in these strata were 

 Melania inquinata, a Vaio, and a MelaTwpsis, but they were not suffi- 

 cient to enable me to determine with precision the age of the formation. 

 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 demarcation. In the cavities of such mixed rocks 

 we often find chalcedony, and crystals of mesotype, stilbite, and arrago- 

 nite. To formations of this class may belong some of the breccias 

 immediately adjoining the dike in the hill of Gergovia; but it cannot be 

 contended that the volcanic sand and scoriae interstratified with the marls 



* See Scrope's Central France, p. 7. 



