142 



Prof. J. Prestwich. 



Sub- 



Appenine 



Strata, 

 continued. 



Eocene. 



fl3. 



14 

 1 15- 

 I 



(16. 



(1 , 



Thickness. Dentil, 



metres. metres. 



Micaceous marl and siliceous 



limestone 7'81 379'70 



Compact micaceous marl . . 53*19 432 90 

 Compact marl with layers of 



limestone 25-72 458'62 



Argillaceous limestone .... 2*00 460'62 

 Macigno (a hard calcareous 



sandstone) 4&th spring 4'08 464" 70 



No ascending spring was met with until the volcanic ashes stratum 

 No. 4 was reached at a depth of 368 feet. Another spring was found 

 at a depth of 830 feet in the Tertiary marly sands No. 9 ; another in 

 the micaceous sands No. 12 at 1106 feet, but no spring of the desired 

 volume was met with until a sandy bed under the Macigno was 

 reached at a depth of 1524 feet. The discharge of water from this 

 bed amounted to nearly 2 cubic metres per minute, and rose about 

 30 feet above the surface, so that the water could be used as a natural 

 fountain in the Palace gardens. In the Piazza the artesian waters 

 formed another natural fountain rising 8 feet above the surface. 



These wells, therefore, show the existence of one important spring in 

 a stratum of volcanic debris, and of three springs in the sedimentary 

 strata beneath. But this only gives the more powerful ascending 

 springs ; bodies of water of lesser volume, or which do not rise to the 

 surface, escape notice in works of this description. 



The overflow of the water from the bed of volcanic ashes proves 

 that the bed comes to the surface at a level higher than that of the 

 ground where the well is situated, and as the water has sufficient 

 ascensional force to rise several feet above the ground, it must neces- 

 sarily, in order to overcome the resistance or friction of the bed 

 through which it passes, stand at its outcrop in the central volcanic 

 area considerably higher than at the point of overflow. There must, 

 therefore, be a continuous sheet of underground water, the level of 

 which rises towards the centre whence the water-bearing bed pro- 

 ceeds. In this way all the permeable beds of a volcanic mountain will 

 be charged with water, the level of the water rising with the distance 

 from the point of escape and with the height of the mountain. Every- 

 where beneath the level of saturation the surface-waters will eventually 

 fill all fissures and interstices, and lodge in them permanently, unless 

 disturbed or drawn off by artificial or natural means (see fig. 1). 



This level depends, when there are alternating permeable and im- 

 permeable beds, on the height of outcrop of the former, and, when the 

 whole mass is permeable, on the texture of the rock, or the friction 

 to which the water is subjected. In the Chalk hills of the south 

 of England which are composed of a comparatively homogeneous 



