254 Notice of Active and Extinct Yolcanos. 



is not tufaceous, but consists of cellular basalt. The occurrence 

 of this substance, sometimes cellular, sometimes amygdaloidal, 

 and sometimes even compact, interstratified with the other rocks, 

 renders the structure of Vicentin less simple than it would other- 

 wise be considered, and inclines one to think that streams of lava 

 were thrown out during- the formation of the tufaceous and cal- 

 careous beds. That the whole indeed of the basaltic, as well as 

 the materials of the tufaceous rocks are referable to igneous ac- 

 tion, I cannot bring- myself for a moment to doubt, although aware 

 that Brocchi, the first of Italian Geologists, has in his Memoir on 

 the Val de Fassa expressed himself with some degree of hesita- 

 tion on the subject." 



" The presence of shells in the tuff itself, and its alternation 

 with regular beds of unaltered shelly limestone, prove that the 

 sandy matter and loose fragments of which this aggregate is com- 

 posed, were originally deposited under the surface of water, at 

 the period during which the calcareous beds were in the act of 

 forming. That the accumulation of the materials of which the 

 tuff consists was a slow and gradual process, I infer among other 

 reasons, from a specimen in my possession, in which a rounded 

 fragment taken from one of these beds is seen covered by serpu- 

 lae, a plain proof that the stone remained for some time under 

 water, uncovered by any of the matter which afterwards formed 

 above it. 



" The occurrence therefore of beds of volcanic tuff alterna- 

 ting with strata of shelly limestones seems in this instance capa- 

 ble of explanation, by supposing showers of ashes and lapilli to 

 have proceeded from some adjacent volcano, which, as they sunk 

 to the bottom of the water then covering the face of the country, 

 would become intermixed with the fragments washed down from 

 the adjoining rocks, and be consolidated like mud in a stagnant 

 pool, acquiring additional consistency in proportion to the mass 

 of matter superimposed. 



" That the volcanic action was indeed going on in this very 

 spot, is proved by the hills of cellular lava, or of basalt, that 

 occur in the midst of this formation, and the effects of these 

 operations upon the tuff itself may be traced in the inclined 

 position of its beds, so different from what would occur in a mass 

 of matter deposited tranquilly under the surface of water. 



As there is probably no locality of ichthyolites more in- 

 teresting than that of Bolca, we will introduce a more par- 

 ticular account and description from M. Brongniart's 316- 

 moire sur les terrains de sediment superieurs — calcareo-trap- 

 peens du Vicentin, of which we have received a copy from 

 the author. 



