SUBMARINE TRAVERTINE 323 



of the Anio, measuring about six miles from east to 

 west and four miles from north to south. Here the 

 quarries of ancient Rome were opened, and the 

 modern city still draws its supplies from the same 

 area. A smaller tract of similar stone has been 

 worked for building material at Cisterna di Roma, 

 about twenty-eight miles to the southeast of the city. 

 A thin band of travertine, possibly a continuation of 

 that at the Torretta di Quinto, forms a conspicuous 

 cliff along the east side of the Via Flaminia, between 

 Ponte Molle and the Porto del Popolo, and other 

 detached masses of it make their appearance at various 

 places further south. 



These various outcrops of travertine, as I shall have 

 occasion to show, have had an important influence in 

 the excavation of the valley of the Tiber and the iso- 

 lation of the hills of Rome. The material was probably 

 deposited chiefly by hot springs containing abundant 

 carbonate of lime in solution, and may be regarded as 

 an accompaniment or sequel of volcanic activity. In 

 most cases, thermal mineral springs depositing traver- 

 tine make their appearance in the later phases of a 

 volcanic period, and often continue long after every 

 other manifestation of subterranean heat has died out. 

 But at the Torretta di Quinto the sheet of travertine 

 is found among the records of the beginning of the 

 volcanic history. Another exceptional feature in this 

 sheet is the proof that it was laid down on the floor 

 of the sea, for it encloses the remains of some of the 

 shells that lived at the time in that sea. 



More direct and obvious proof of the breaking-out 

 of volcanic eruptions is to be found in the strata that 



