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Show FALLS OF NIAGARA. 182 h' . t £ t in thickness, stretc mg of hard limestone, a~outd~me .Y ee er the whole country, . h · ntal 1rect10n ov . nearly m a orlZO . bove the falls, as do the mfe-and forming the bed of tlhe nverl al t's nearly of the same thick- . h 1 b l '"fhe ower s 1a e . . . rlOr s a es ~ .ow. Should Lake Erie remam m Its present ness as t~e hmest~:n~ when the ravine recedes to its shores, the state until the peno b d of water would cause a tresudden escape of tl~at ghreat .o ywould be much more than suf-d d 1 ge . 1or t e ravme men ous e ~ ' h 1 I k of which the average depth £cient to dram the w lote a e,e to be only ten or twelve .1! d d ing the a e surv Y, . was toun , ur of 1• ts shallowness, Lake Er1e h B t in consequence fat oms. . u ' . a· t and the annual growth of the is fast fillmg up .with sedt~en 'ts which flow into it is remarkdeltas of many r~vers.L'an orrere near the influx of Big Creek bl L ng Pomt, lOr examp ' l a . e. 0 , ed durin the ]ate survey, to advance t nee Rtver, was observ ' A g uestion therefore arises, whether miles in as many yeabrs. qted l'nto dry land before the Falls L k E.· may not e conver . ofa NeI. agnarea rece d e so .l1!a r . In speculating on this cobnlt mgencAy , we must not oml' t one l'mportant con.d itio· n ofh t h1e pro f em.t b s h I ke is contracted m stze, t e oss o wa er y the surfa~e of 'tll ~l·mai.nish. and unless the supply shall decrease evaporatiOn w1 u ' bl ) N' st in the same ratio (which seems scarcely proba. e ' Ia~ara mtu augment continually m. vo l ume, an d by thisI meadns Its re ro-grad e movemen t may hereafter be much acce erate . CHAPTER XI. Action of running water, continued-Course of the Po-Desertion of its old channel -Articial embankments of the Po, Adige, and other Italian rivers-Basin of the Mississippi-Its meanders-Islands-Shifting of its course-Raft of the Atchafalaya-Drift wood-New-formed lakes in Louisiana--Earthquakes in the valley of tho Mississippi-Floods caused by land-slips in the White mountains-Bursting of a lake in Switzerland-Devastations caused by the Anio at Tivoli. Course of the Po.-TnE Po affords a grand example of the manner in which a great river bears down to the sea the matter poured into it by a multitude of tributaries descending from lofty chains of mountains. The changes gradually effected in the great plain of Northern Italy, since the time of the Roman republic, are very considerable. Extensive lakes and marshes have been gradually filled up, as those near Placentia, Parma, and Cremona, and many have been drained naturally by the deepening of the beds of rivers. Deserted river-courses are not unfrequent, as that of the Serio Morto, which formerly fell into the Adda, in Lombardy; and the Po itself has often deviated from its course. Subsequently to the year 1390, it deserted part of the territory of Cremona, and invaded that of Parma; its old channel being still recognizable, and bearing the name of Po Morto. Bressello is one of the towns of which the site was formerly on the left of the Po, but which is now on the right bank. There is also an old channel of the Po in the territory of Parma, called Po Vecchio, which was abandoned in the twelfth century, when a great number of towns were destroyed. There are records of parish-churches, as those of Vicobellignano, Agojolo, and Martignana, having been pulled down and afterwards rebuilt at a greater distance from the devouring stream. In the fifteenth century the main branch again resumed its deserted channel, and carried away a great island opposite Casalmaggiore. At th~ end of the same century it abandoned, a second time, the |
