136 



Prof. J. Prestwich. 



the superficial volcanic strata will usually flow towards the circum- 

 ference of the mountain, in consequence of the beds by which they 

 are held up dipping from the central crater ; while the springs in the 

 sedimentary strata lie in the continuous planes traversed by the vol- 

 canic duct, and towards which they may often dip. Some irregularity 

 in the phenomena is therefore to be expected, and while in most 

 places the wells suffer, it is quite intelligible that in others they may 

 be but little affected. 



Fig. 1. — Diagram section of a volcano. The dark bands represent lava-streams ; 

 the dotted spaces, ashes and scoriae j and the line I, the water level in the mountain. 



As bearing upon the subject of the disturbances to which water- 

 bearing strata are liable in volcanic districts, we have the evidence of 

 M. Mauget, a well- engineer of great experience, respecting the 

 sudden changes of water level in the Neapolitan area, during his 

 residence there. There were eruptions of Vesuvius in 1865 and 1867, 

 but none in 1866. Nor were there any important earthquakes, but a 

 number of minor ones are recorded. 



M. Mauget says that in May, 1866, the wells and springs around 

 Naples began to be affected, and continued to diminish until June, 

 but considers that this might be due to ordinary causes, such as lesser 

 rainfall. On the 29th June a sudden change took place. The waters 

 of the aqueduct, which brought in the water from a distance of 

 12 miles, and of the canal of Lagno di Mofita, as well as of various 

 rivers, became troubled and reduced in a surprising manner. The next 

 day the waters became bright, but were found to be reduced to the 

 extent of one-fifth of their volume. The great springs of the Sannio 

 district were reduced by one-third ; and the town of Sorrento was de- 

 prived of all potable water. This water is brought in by an aqueduct 

 from the neighbouring hills, which consist of Eocene or of Cretaceous 

 strata. The whole district, from the foot of the Apennines to the 

 Neapolitan coast, was affected over an area of 66 miles square. 



At the same time various artesian wells in the valley of Sebito 

 became sanded up and greatly reduced in their flow, and the two deep 

 artesian wells of Naples threw up above 200 cubic metres of trachytic 

 and pumiceous sands and lapilli.* 



It is evident that this diminution in the surface waters could only 



* " Sur les variations subites dans le regime de divers cours d'eau dans 1'Italie 

 Meridionale," " Comptes rendus," vol. lxiv, p. 189 (1867). 



