110 NEWER TLIOCENE PERIOD. [Ch. IXi' 
annually to deepen the ravines in which they flow, and the tra- 
veller occasionally finds that the narrow mule-path, instead of 
winding round the head of a ravine, terminates abruptly in a 
deep trench which has been hollowed out, during the preceding 
winter, through soft clay. But throughout a great part of 
Italy, where the marls and sands of the Subapennine hills are 
elevated to considerable heights, the same rapid degradation is 
often perceived. 
In the limestone districts of the Val di Noto, the strata are 
for the most part nearly horizontal, and on each side of the 
valley form a succession of ledges or small terraces, instead of 
descending in a gradual slope towards the river-plain in the 
manner of the argillaceous formations. When there is a bend 
in the valley, the exact appearance of an amphitheatre with a 
range of marble seats is produced. A good example of this 
configuration occurs near the town of Melilli, in the Val di 
Noto, as seen in the annexed view (No. 22). In the south of 
No. 22. 
Valley called Gozzo degli Martiri, lelow MelilU. 
the island, as near Spaccaforno, Scicli and Modica, precipitous 
rocks of white limestone, ascending to the height of five hun- 
dred feet, have been carved out into the same form. 
A careful examination of the mode of decomposition of the 
rock would be requisite, in order fully to explain this pheno- 
menon. There is probably a tendency to a vertical fracture in 
