DOMAIN V. CALCAREOUS. 



cements, that the exportation of puzzolana, by 

 the Tiber and the Port of Ostia, is become a 

 little branch of trade."* 



From the preceding observations, it might 

 be understood that there are at least three dif- 

 ferent structures of the calcareous tufa : the 

 porous, or that of the travertine ; the shelly, like 

 that of the ruins of the temple of Jupiter; and 

 the tubular, like that of Germany, and the north 

 of England. 



STRUCTURE I. POROUS. 



Travertine, from the ruins of Rome. 

 The same, from the quarries near Tivoli. 



STRUCTURE II. CONCHIT1C. 



From the ruins of Agrigentum or Girgenti. It 

 is found in many other places, but has excited 

 little attention, being probably regarded as a 

 coarse limestone ; while its lax composition refers 

 it to this mode, though Wallerius would perhaps 

 have called it a-tufaceous limestone. 



* Breislak, ii. 26 1. At Bionnay there are houses built of a cal- 

 careous tufa, containing fragments of lime-spar, limestone, and 

 slate; the base being of a lively brick red colour, and strongly effer- 

 vescent with acids. Sauss. 752. 



