33o THE ROMAN CAMPAGNA 



three craters which have risen through each other to 

 the north of Astroni, that some of the Neapolitan 

 vents were only a few yards in diameter. And we 

 learn also that at least one, and probably others of 

 them, were the product of single eruptions, for Monte 

 Nuovo, which is nearly 500 feet in height, was thrown 

 up in the course of two days. Doubtless, these small 

 and rapidly built monticules had many predecessors 

 of like type on the Roman Campagna. 



In the second place, the cones connected with the 

 tuff of the district around Rome, being composed of 

 loose fragmentary materials, would be easily washed 

 down. No one can ramble over that area without being 

 struck with the singular scarcity of solid lava among the 

 endless exposures of tuff. It is true that around the 

 great craters of the Alban and Ciminian Hills a good 

 deal of lava can be seen to have been emitted. But 

 these masses, like the volcanoes that gave vent to 

 them, belong to that later stage of the volcanic history 

 to which I have referred. Only to a trifling extent 

 does the tuff of the Campagna appear to include con- 

 temporaneous sheets of lava. If, then, molten rock 

 has hardly ever poured out at the surface, it may 

 rarely have risen and consolidated in the upper parts 

 of the throats of the volcanoes, so as to form there 

 a hard core which would remain as a projecting knob 

 when the surrounding loose ashes were levelled down 

 by denudation. 



In the third place, there can now be no doubt that 

 the greater part of the sheet of tuff in the Roman 

 Campagna was accumulated under the sea. This 

 subject was for many years one on which various 



