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Show 90 NEWER PLIOCJ~NE PERIOD. [Ch.VII. . 1 5 serve by contrast, to here and there escape d tl1 e burmno0- ava ' When' I visited the d 1 t · of the scene. heighten the eso a 1011 • f IS19 I saw hundreds of valley, nine years after the ekrul ptwn of trees' on the borders of h 1 1 ite 5 e etons o ' trees, or rat er t 1e w 1 d b ches beinO' all leafless, and 1 h t unks an ran o the black ava, t e r 1 h'ng heat emitted from the . f 1 . b ·k by t 1e score 1 depnved o t lelr ai . b tifullines-melted rock ; an image recallmg those eau - ' As when heaven's fire . . llath scath' d the forest oa k s, or mountam pmes, With singed top their stately growth, though bare, Stands on the blasted heath.' Form composz. tw. n, an d or igin 0'J1 ' the Dik• es• .-But without indul in' g the I. magm. a t'w n a ny lonOo 'er in desc. np. twns of scenery, we mga y observe, t h a t tl1 e dikes befo.r e mentwned form· unhq ues-l tionably the most interesting geological phenomenon m t eVa del Bove. No.l9. Di!.cs at t!te base of the Si'rre del Su!jizio, Etna. Some of these are composed of trachyte, others ot compact Ch. VII.] DIKES ON ETN t\, 91 blue basalt with olivine. They vary in breadth from two to twenty feet and upwards, and usually project from the face of the cliffs, as represented in the annexed drawing (No. 19). They consist of harder materials than the strata which they traverse, and therefore waste away less rapidly under the influence of that repeated congelation and thawing to which the rocks in this zone of Etna are exposed. The dikes are, for the most part, vertical, but sometimes they run in a tortuous course through the tuffs and breccias, as represented in diagram, No. ~0. In the escarpment of Somma where, as we No. 20. --------------.__ J7eins of Lava. Punto di Guimenlo, before observed, similar walls of lava cut through alternating beds of sand and scorire, a coating. of coal-black rock, approaching in its nature and appearance to pitch-stone, is seen at the contact of the dike with the intersected beds. I did not observe such parting layers at the junction of the Etnean dikes which I examined, but they may perhaps be discoverable. The geographical position of these dikes is most interesting, as they occur in that zone of the mountain where lateral eruptions are fre<}Uent ; whereas, in the valley of Calanna, which is below that parallel, and in a region where lateral eruptions are extremely rare, scarcely any dikes are seen, and none whatever still lower in the valley of St. Giacomo. 'l'his is precise] y what we should have expected, if we consider the vertical fissures now filled with rock to have been the feeders of lateral |