126 M. Dafrenoy on the Volcanic Formations 



feet above the level of high water, and M. Challey's 167 feet 

 above the level of the River Sarine, 



Comparisons with certain points in the city of Paris give a 

 more lively idea than any numbers of the magnitude of the work. 

 Only conceive a bridge of one arch as long as the raihng of the 

 Carrousel, or the distance between the wickets leadino- to the 

 galleries, and a roadway as high as the Towers of Notre Dame, 

 or the column in the Place Vendome, and you may have some 

 idea of the bridge of Fribourg. 



On the Volcanic Formations of the Environs of Naples. By 

 M. DuFRENoy. 



M. DuFRENOY, in a memoir read to the French Academy of 

 Sciences on the 18th of November 1835, describes successively 

 the deposit of pumice tuff of which the Campania of Naples is 

 composed ; and the nature and formation of the hills of the Phle- 

 grjean Fields and the group of Vesuvius, in which latter he dis- 

 tinguishes the Somma and Vesuvius properly so called. He termi- 

 nates his memoir with various considerations regarding the phe- 

 nomena which produced the destruction of Herculaneum and 

 Pompeii. In order to give an idea of the labours of M. Dufrenoy, 

 we shall transcribe the conclusions at which he arrives, and 

 which include the principal results of his investigations. 



Different Epochs of the Volcanic Phenomena, 



1. The igneous phenomena have manifested themselves in the 

 vicinity of Naples at three periods very distant from one an- 

 other, and with different degrees of intensity, and with different 

 characters. 



The first period, and of which the geological epoch is un- 

 known, is marked by the deposition of the trachytes, which 

 have afforded the elements of the pumice-tuff; the lavas of the 

 Somma occurring in horizontal masses, and also the Jeucitic rocks 

 of the environs of Rome. 



The production of the trachytes of the Phlegrgean Fields and 

 of Ischia took place in the second period. 



The third period includes the lava eruptions of Ischia, Ve 

 suvius, and Monte Nuovo. 



