Ch. XXXII] 



GERGOVIA. 



693 



where a freshwater marl alternates with volcanic tuff containing 

 Miocene shells. The tuff or breccia in this locality is precisely such 

 as is known to result from volcanic ashes falling into water, and sub- 

 siding together with ejected fragments of marl and other stratified 

 rocks. These tuffs and marls are highly inclined, and traversed by 

 a thick vein of basalt, which, as it rises in the hill, divides into two 

 branches. 



Gergovia. — The hill of Gergovia, near Clermont, affords a third 

 example. I agree with MM. Dufrenoy and'Jobert that there is no 

 alternation here of a contemporaneous sheet of lava with freshwater 

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

 position and contents of some of the associated tuffs prove them to 

 have been derived from volcanic eruptions which occurred during the 

 deposition 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 Merdogne. The dike here cuts through the marly strata at a con- 

 siderable angle, 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, con- 

 taining freshwater shells, are seen to alternate with volcanic tuff. In 

 the lowest part of this division, beds of pure marl alternate with com- 

 pact fissile tuff, resembling some of the subaqueous tuffs of Italy and 

 Sicily called pejperinos. Occasionally fragments of scoriss are visible 

 in this rock. Still higher is seen another group of some thickness 

 consisting exclusively of tuff, upon which lie other marly strata inter- 4 



White 

 and green 

 marls. 



Hill of Gergovia. 



mixed with volcanic matter. Among the species of fossil shells which 

 I found in these strata were Melania inquinata, a Unio, and a Mela- 



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



