318 THE ROMAN CAMPAGNA 



the Monte Verde, south of the Janiculan ridge. The 

 clays and sands which rise in horizontal layers almost 

 to the top of Monte Mario are there entirely cut 

 out, and the tuff, which lies as a mere thin capping 

 on the crest of that hill, suddenly descends across 

 the truncated edges of the Pliocene strata to the 

 alluvial plain of the Tiber. On the opposite side of 

 the river, the upper sands have been almost entirely 

 removed and the tuff is found lying almost immedi- 

 ately on the lower clay. It is clear that there must 

 have been an extensive, though no doubt local, erosion 

 of these marine strata, before the main body of the 

 tuff was laid down. By what agency this erosion 

 was effected is not quite clear. Not improbably a 

 gap occurs here in the record, representing an interval 

 of considerable duration of which no chronicle has 

 survived. 



Passing over this hiatus, we still encounter marine 

 deposits, but these are of volcanic origin, and leave 

 us meanwhile to speculate in the dark as to whether 

 the denudation was the work of the sea or of 

 terrestrial waters. Owing to the thick covering of 

 tuff which has overspread the Campagna and con- 

 cealed all that lies below, it has become difficult to 

 obtain adequate data for the discussion of this ques- 

 tion, which is of considerable interest in the history 

 of the Campagna. The most reliable evidence would 

 be supplied by a series of borings across the district 

 in different directions. Such a series may perhaps 

 hereafter be undertaken for the purpose of obtaining 

 water for the farms and homesteads, which sanguine 

 patriots foresee taking in the future the place of the 



