OCR Text |
Show 110 NEWER PLIOCENE PERIOD. (Ch.IX: annually to deepen the ravines in which they flow' an~ the tra-ve Il er occasw· na1 1 y fi nd s that the narrow mule-pathb, mstead of wm· dm· O' roum1 t 11 e 11 ea d of a rav 1'ne , terminates a ruptly in a deep tJ~nch which has been hollowed out, during the preceding winter throucrh soft clay. But throughout a great part of Italy, 'where cl1e marls and sands of the Sub~pennine hil!s m:e elevated to considerable heights, the same rap1d degradatiOn Is often perceived. . In the limestone districts of the Val d1 N oto, 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 rancre 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. ~~). In the south of No.2 ~ . Valley called Goz:eo degli Martil'i, Lolow Mcliili. the island as near Spaccaforno, Scicli and Modica, precipitous rocks of ~bite 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 fractul'e in Ch. IX.] PROOFS OF SUCCESSIVE ELEVATION. 111 this as in many other limestones, which, when exposed to the action of frost, scale off in small fragments at right angles to the plane of stratification. It might have been expected that, in this case, a talus composed of a breccia of the limestone would be found on each ledge, so that the slope would become gradual, but perhaps the fragments instead of accumulating may decompose and be washed away by the heavy rains. The line of some of the valleys near Lentini has evidently been determined mainly by the direction of the elevatory force, as there is an anticlinal dip in the strata on either side of the valley. The same is, probably, the case in regard to the great valley of the Anapo, which terminates at Syracuse. Sea-cliffs-proofs of successive elevalion.-No decisive evidence could be looked for in the form of the valleys to determine the question, whether the subterranean movements which upheaved the newer Pliocene strata in Sicily were very numerous or few in number. But we find the signs of two periods of elevation in a long range of inland cliff on the east side of the Val di Noto, both to the north of Syracuse, beyond 1\'Ielilli, and to the south beyond the town of Noto. The _great limestone formation before mentioned, terminates suddenly towards the sea in a lofty precipice, a, b, which varies in height from five No. 23. ' t-'·------··----·--·---·--·-·----··---······-·· ·;}}1, hundred to seven hundred feet, and may remind the English geologist of some of the most perpendicular escarpments of our chalk and oolite. Between the base of the precipice a.' ~' and ~he .sea, is a-n inferior platform, c, b, consisting of Slmllar white hmestone. All the strata dip towards the sea, |