OCR Text |
Show 108 NEWER PLIOCENE PERIOD• [Ch.IX. into tuffs and peperinos, nor can we imagine that,. under enormous pressure, they could have become porous, smce we observe, that the lava which has cooled down und_er a .moderate degree of pressure, in the dikes of Et~a and Vesuvius, has a compact and porphyritic texture, and 1s ~ery rar~ly porous or cellular. No sicrns of volcanic sand, scor1re, breccia, or conglomerate are to b; looked for, nor any of stratification, for all these imply formation in the atmosphere, or by the agency of water. The only proofs that we can expect to find of the successive oricrin of different parts of the fused mass, will be 0 . confined to the occasional passage of vems through portions previously consolidated. This consolidation would take place with extreme slowness, when nearer the source of volcanic heat and under enormous pressure, so that we must anticipate a perfectly crystalline and compact texture in all these subter~ ranean products. Now geologists have discovered, as we before stated, great abundance of crystalline and unstratified rocks in various parts of the globe, and these masses are particular! y laid open to our view in those mountainous districts where the crust of the earth has undergone the greatest derangement. These rocks vary considerably in composition, and have received many names, such as granite, syenite, porphyry, and others. That they must have been formed by igneous fusion, and at many distinct eras, is now admitted; and their highly crystalline texture is such as might result from cooling down slowly from an intenselyheated state. They answer, therefore, admirably to the conditions required by the above hypothesis, and we therefore deem it probable that similar rocks have originated in the nether regions below the island of Sicily, and have attained a thickness of from one thousand to three thousand feet, since the newer Pliocene strata were deposited. It is, moreover, very probable, that these fused masses have come into contact with subaqueous deposits far below the sur· face, in which case they may, in the course of ages, have greatly Ch.IX.] FORMS OF THE SICILIAN VALLEYS, 109 altered their structure, just as dikes of lava render more crystalline the stratified masses which they traverse, and obliterate all traces of their organic remains. Suppose some of these changes to have been superinduced upon subaqueous deposits underlying the tertiary formations ~f Sicily, it is important to reflect that in that case no O'eolocrical 0 0 proofs would remain of the era when the alterations had taken place; and if, at some future period, the whole island should be uplifted, and these rocks of fusion, together with the a1tered strata, should be brought up to the surface, it would not be apparent that they had assumed their crystalline texture in the newer Pliocene period. For aught that would then appear, they might have acquired their peculiar mineral texture at epochs long anterior, and might be supposed to have been formed before the planet was inhabited by living beings; instead of having originated at an era long subsequent to the introduction of the existing species. CHANGES OF THE SURF ACE DURING AND SINCE THE EMER-GENCE OF THE NEWER PLIOCENE STRATA. Vall:ys.-Geo1~gists ~~o are accustomed to attribute a great portiOn of the mequahues of the earth's surface to the excavating power of running water during a long series of ages, will probably look for the signs of remarkable freshness in the aspect of cou.ntries so recently elevated as the parts of Sicily already described. '!'here is, however, nothincr in the external configurati.on of that country which would strike the eye of the most practised observer, as peculiar and distinct in character fro~ l~any other distrjcts in Europe which are of much higher antiqmty. The general outline of the hills and valleys would accord perfectly well with what may often be observed in regard to other regions of equal altitude above the level of the sea. It is .true that, towards the central parts of the island where the ~rgtllaceous deposits are of great thickness as around Cas-trocor to vanm·, ca 1 t ams· etta~ and Piazza, the torre' nts are observed |