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Show EROSION OF RAVINl£S., 176 . d' . us burden of rock anu ~f: pro JO'lO d b down upon their sur ace a . ol are generally arrange ear . Tl sc matena s . ~ ~ d mixed with 1ce. 1e. . l Alps are th1rty or .1orty ::san h' h metimes m t le . l'k . lon(J' rid(J'es, w IC so h b ders of the glacier, 1 e so m o b • allel to t e or d b . ~.1 eC t ht' bO 'h ' runmn• g par } t Tl1 ese mounds of e r1s are many lines of mtrenc H~en . nd have generally been brought sometimes three or more eeph ~ accumulation is slowly conin by lateral glaciers: the w : ee on the melting of the gla-d to the lower valleys, w er ' veye b rivers*· 11 cier it is swept a':ay Y. 1 the smallest streams ho ow The rapidity Wl~h whlC 1 n~ve~estructible soils is remarkably out deep channels m soft a . here the sand and half-con- . 1 ic countnes, w exemplified m vo can t a sliCTht resistance to the torre~ts solidated tuffs oppose bu . .Ode After the heavy rams d h mountam Sl • h t which descen t e . f Vesuvius is 18~~, t e wa er which followed the er.updtiOlnCo llo cut in three days a new . h Atno e ava ' . . t flowmg from t e ff d volcamc eJected matter o 1 t t of tu an . chasm throug 1 s ra a £ The old mule road was seen, m the depth of twenty-five ~et. 'ne But such facts are d b this new ravi . d 18~8 ' intersecte Yd to 1 . t CTorcres which are excavate t 1e gt ea b n · triflin(J0 ' when compare . 1 . the great plateau of Mexico, • '1 . materia s m f in somewhat simi ar , f lleys orio·inally worn out o where an anci. en t sy stem o hv a b 'n subbs equently fi ll ed wl't h d d y rocks, as ee th granite an secon ar d trachytic conglomerate, to e strata of tuff, pumice, lava,:~ feet. The rivers and torren.ts thickness of several th?u;a 'ns are now actively employed m annually swoln by tropica ·r~ ~sits and in re-excavating the remov.m g tl1ese more recent ep ' ancient water-course.s 1-. d chasms through some of the The gradual erosiOn of eep f running water charged hardest rocks, by the constant pashsage o non of which striking Wl · ther p enome . 'th foreie:n matter, IS ano f tl clearest illustratiOns "" . dd d Some o Je , . examples may b~ a uce . resented by many valleys m of this excavatmg pow~r ~e ~els of rivers have been barre~ Central France, where t e c an ucrh which the streams have up by solid currents of lava, t~ro th of from twenty to seventy re-excavated a passage to the ~P reat width. In these cases feet and upwards, and often o ~ . d 1 Alpes tom. 1. esearcbes In • Saussure, Voyage ans :s t h for this information, whose r t I am indebted to Captam Ve c . ted to the scientific world. Mexico will, it is hoped, be soon commumca AQUEOUS EROSION OF LAVAS. 177 there a1·e decisive proof.c; that neitl1er tl1e sea nor any denuding wave, or extraordinary body of water, have passed over the spot since the melted lava was consolidated. Every hypothesis of the intervention of sudden and violent ag.ency is entirely excluded, because the cones of loose scorioo, out of which the lavas flowed, arc oftentimes at no great elevation above the rivers, and have remained undisturbed during the whole period which has been sufficient for the hollowing out of such enormous ravines. But we shall reserve a more detailed account of the volcanic district of Central France for another part of this work, and at present confine ourselves to examples derived from events which have happened since the time of history. Some lavas of Etna, produced by eruptions of which the date is known, have flowed across two of the principal rivers in Sicily; and in both cases the streams, dispossessed of their ancient beds, have opened for themselves new channels. An el'uption from Mount Mojo, an insulated cone at the northern base of Etna, sent forth, in the year 396, n.c., in the reign of Dionysius I., a great lava-stream, which crossed the river Cal .. · tabianca in two places. The lowermost point of obstruction is seen on the eastern side of E~na, on the high road from Giardini to Catania, where one pier of the bridge on either bank is based upon a remnant of the solid lava, which has been breached by the river to the depth of fourteen feet. But the Caltabianca, although it has been at work for more than two and twenty centuries, has not worn through the igneous rock so as to lay open the gravel of its ancient bed. The declivity, however, of the alluvial plain is very slight; and as the extent of excavation in a given time depends on the volume and velocity of the stream, and the destructibility of the rock, we must carefully ascertain all these circumstances before we attempt to deduce from such examples a measure of the force of running water in a given period#..'. . " I omitted to visit the higher point near the village of Mojo, wl1ere the CaltabJanca has cut through the lava. Some future traveller would probably derive much instmction from inspecting that spot, which is laid down in GemmeUaro's Qunuro Istorico, &c. dell' Etna, 1824. Vot, I. N |