TUFFS OF MARINE ORIGIN 327 



and animals have obviously been drifted from the land, 

 and did not live where their remains have been found. 

 It is worthy of remark that though so many observers 

 have been at work in successive generations among the 

 rocks of the Campagna, no undoubted example of 

 marine mollusk has been recorded from the tuff in the 

 interior of the Campagna. The crowds of shells in the 

 Pliocene strata underneath are there absent. That the 

 conditions required for the existence of an abundant 

 marine fauna continued over this site until the 

 beginning of the volcanic period is manifest from the 

 crowded pteropods, lamellibranchs and gasteropods of 

 the clays and sands. But as the eruptions increased 

 in area and in intensity these conditions were eventually 

 destroyed. The descent of continued showers of hot 

 dust, ashes and stones over the sea-bottom, the rise of 

 mephitic gases from below, as well as of hot springs 

 that deposited sheets of travertine, must have made 

 that sea-floor no congenial home for either plant or 

 animal. 



It has often been assumed that the tuff of the 

 Roman Campagna was derived from the eruptions of 

 the Alban volcano on the one side, and of the Bracciano 

 volcano on the other. A careful study of the tuff, 

 however, and a comparison of it with that of more 

 ancient volcanic districts, the structure of which has 

 been more fully laid open by prolonged denudation, 

 leads, in my opinion, to a conviction that this assump- 

 tion is founded on inaccurate observation. The rapidly 

 varying and lenticular character of the materials when 

 followed along the cliffs where they are exposed, and 

 their occasional agglomeratic character which increases 



