OCR Text |
Show ( 62 ) CHAPTER VI. • R £or considering in the first place the more PI' ene formahons- easons Newer lOC • 1 . 1 ·t. ture of Sicily-Formations of the Val di N oto odern perwcls-Geo ogica s we . m . . d p· · ible into three groups-Great limestone-of newer Pliocene peno - !VIS • l. t e-Blue marl with shells-Strata subjacent Schistose aud arenaceous lmes on . . · k f the Val di Noto-Dikes-Tu:ffs and Pe1)ermos to the above-Volcamc roc s o . . _ v . t -Proofs of long intervals between volcamc eruptions olcamc conglomera es . . -Dip and direction of newer Pliocene strata of Sicily. NEWER PLIOCENE FORMATIONS. HAVING endeavoured, in the last chapter, to explain the principles on which the different tertiary formations may be. arranged in chronological orde~, we shall now ~roceed to consider the newest division of formatiOns, or that wh1ch we have named the newer Pliocene. It may appear to some of our readers~ that we . r~vers.e the natural order of historical research by thus descr1bmg, m the first place, the monuments of a period which immediately preceded our own era, and passing afterwards to the events of antecedent ages. But, in the present state of our science, this retrospective order of inquiry is the only one which can conduct us gradually from the known to the unknown, from the simple to the more complex phenomena. We have already explained our reasons for beginning this work with an examination, in the first two volumes, of the events of the recent epoch, from which the greater number of rules of interpretation in geology may be derived. The formations of the newer Pliocene period will be considered next in order, because these have undergone the least degree of a1teration~ both in position and internal structure, subsequently to their origin. They are monuments of which the characters are more easily deciphered than those belonging to more remote periods, for they have been less mutilated by the hand of time. The organic re~ains, more Ch. VI.] GEOLOGICAL STRUCTURE OF SICILY. 63 especially of this era, are most important, not only as being iri a more perfect state of preservation, but also as being chiefly referriblc to species now living; so that their habits are known to us by direct comparison, and not merely by inference from analogy~ as in the case of extinct species. Geological st-ructure of Sicily.-We shall first describe an extensive district in Sicily, where the newer Pliocene strata are largely developed, and where they are raised to considerable heights above the level of the sea. After presenting the reader with a view of these formations, we shall endeavour to explain the manner in which they originated, and speculate on the subterranean changes of which their present position affords evidence. The island of Sicily consists partly of primary and secondary rocks, which occupy, perhaps~ about two-thirds of its superficial area*, and the remaining part is covered by tertiary formations~ which are of great extent in the southern and central parts of the island, w bile portions are found bordering nearly the whole of the coasts. Formations of the Val di Noto.-If we first turn our attention to the Val di Noto~ a district which intervenes between Etna and the southern promontory of Sicily, we find a considerable tract, containing within it hills which are from one to two thousand feet in height, entirely composed of limestone, marl, sandstone, and associated volcanic rocks, which belong to the newer Pliocene era. The recent shells of the Mediterranean abound throughout the sedimentary strata, and there are abundant proofs that the igneous rocks were the produce of successive submarine eruptions, repeated at intervals during the time when the subaqueous formations were in progress. These rising grounds of the Val di Noto are separated from the cone of Etna~ and the marine strata whereon it rests, by the low level plain of Catania, just elevated above the level of the sea, and watered by the Simeto, The traveller who passes • We may shortly expect a full account of the Geology of this island from Professor Hoffmann1 who has devoted more than a. year tci its examination. |