OCR Text |
Show E Luc's NATURAL CHRONOMETERS. 302 D" d · .. ous parts of th e great Libyan deserts, and drift san m vat 1 11 f large dimensions had been have shown whether anly va ~~s oted the progress of the sands, h lonO' t 1ese an es . :filled up,- ow o d h d the whole advanced smce the and how ~ar the floof . a 1 up~~nce of all these necessary ele~ . f l tory I m t le a . times o 11s · ' . th doctrines of tlus author, respect· ments of the computatwn, "e re extremely popular, and that, ing" natural chronomleters, wening and rigorous investigation . e when c ose reaso . . too, m an ~g other branches of physical sCience, lt ~n 1y were applied to the prepossessions in regard to time roves how strong were P . d d h progress of geology. which 1mpe e t e tion relating to the former changes There lS not one. gr~atl qub~ts ts l'nto which considerations of 1 d 1ts m 1a 1 an . of the eart 1 an d 1 s the public mind was viOlently time do not. enter; an s~. on.g a ortant topic, men of superior prejudiced m reg~;!~~h\ ~~rl~~mselves, and were not blinded talent alone, who d d just conclusions from geoloby authority, could ehuce atnytherefore to be matter either g1. l 'd It oug t no ' ' ca ev1 . ence. d' uraO'ement t o us, that at the commence-of surpnse or lSCO o h for three hundred years f t1 present century, w en . ~ ment o le d t d to these investigatiOns, so leW h 1 b . had been eva e . msouucn d aan do men h. ghtened vi.e ws h a d met with general receptwn. CHAPTER XVII. R!!productive effects of Tides and Currents-SiHing up of Estuaries does not com· pensate t.he loss of land on the borders of the ocean-Bed of the German Ocean-Composition anu extent of its sand-banks-Strata formed by cur. rents on the southern and eastern shores of the Mcditerranean-Transporta. tion by currents of the sediment of the Amazon, Orinoco, and MississippiStratification- Concluding remarks. Fnolr the facts enumerated in the last chaptet·, it' will appear that, on the borders of the ocean, currents co-operating with tides are most powerful instruments in the destruction and transportation of rocks ; and as numerous tributaries discharge their alluvial burden into the channel of one great river, so we find that many great rivers often deliver their earthy contents to one marine current, to be borne by it to a distance, and deposited in some deep receptacles of the ocean. The current not only receives this tribute of sedimentary matter from streams draining the land, but acts also itself on the coast, as does a river on the cliffs which bound a valley. The course of currents on the British shores is ascertained to be as tortuous as that of ordinary rivers. Sometimes they run between sandbanks which consist of matter thrown down at certain points where the velocity of the stream had been retarded; but it very frequently happens, that as in a river one bank is made of alluvial gravcJ, while the other is composed of some hard rock constantly undermined, so the current, in its bends, strikes here and there upon a coast which then forms one bank, while a shoal under water forms the other. If the coast be composed of solid materials, it yields slowly, or if of great height, it does not lose ground rapidly, since a large quantity of matter must then be removed before the sea can penetrate to any distat: ce. But the openings where rivers enter are generally the pomts of least resistance, and it is here, therefore, that the ocean makes the widest and longest breaches. But a current alone cannot shape out and keep open an |