328 THE ROMAN CAMPAGNA 



and diminishes in various directions, with no reference 

 to the two great volcanic centres on each border of the 

 district, point not to showers of detritus from these 

 centres, or from any other vents at a distance, but to 

 local eruptions from many and generally small vents, 

 discharging here fine, there coarse materials, at different 

 times and independent of each other. I have not my- 

 self been fortunate enough to detect a c neck, 7 which 

 would mark the site of one of these vents, nor so far 

 as I am aware, has any example of this structure been 

 recorded from the general body of the Campagna tuff. 

 But this failure of proof, I am disposed to believe, is 

 to be accounted for rather from the special kind of 

 evidence required not having hitherto been recognised, 

 or searched for with sufficient experience, than because 

 it does not exist. 



One of the tasks which I think might hopefully 

 be undertaken in regard to the geological history of 

 this district is that of seeking for proofs of the dis- 

 tribution of some of the vents whence the tuff was 

 ejected. Among the numerous crags along the hill- 

 sides, and in the abundant stream-courses or fossi y 

 where the naked rock has been laid bare all over the 

 Campagna, sections might be met with that would 

 help to solve this problem. The numerous unques- 

 tionable c craters ' of the Alban and Ciminian Hills 

 belong to a much later stage of the volcanic period 

 than that in which the main mass of tuff" was formed. 

 We must remember also, in considering this question, 

 that the tuff, with its distinctive and persistent charac- 

 ters, stretches far beyond the limits within which the 

 materials fell that were discharged from the Alban or 



