OF THE EASTERN BORDERS. 313 



extend over great areas in Ireland, where they occupy one-tenth of 

 the surface, with a depth in some cases of 40 feet. 



Peat deposits pass under the sea at North Sunderland, Newton, 

 and Howick. At Hartlepool, we found one 6 feet thick, containing 

 many oak-trees covered over with a bed of silt or mud, in which are 

 entombed great numbers of marine shells identical with those now 

 living along the shores of the German Ocean. Within, therefore, 

 a comparatively recent period, there has been a change of level along 

 the Eastern coast ; a forest of trees waved their branches where the 

 billows of the ocean now roll, presenting a type, indeed, of the 

 changes which occurred during the Carboniferous era. 



The peat deposit at the Black Lough, a few miles west of Alnwick, 

 offers another not uninteresting illustration. It occupies a basin, of 

 which the Lough forms a part, and is, in some places, 1 2 feet thick ; 

 at the bottom are stumps of trees, their tops broken off, but still 

 standing rooted in the sandy soil beneath the peat. If this basin 

 were depressed, and detrital matter, brought by water, deposited, that 

 peat, ages hereafter, would be converted into coal, and would present, 

 on a small scale, analogies to a coal-field. 



In countries where vegetation is more rank than in Britain, we have 

 climatal and other conditions more resembling those of the Carboni- 

 ferous era. An instructive example is furnished by the low grounds 

 bordering the Gulf of Mexico. The delta and alluvial plain of the 

 Mississippi have an area of about 30,000 square miles, being some- 

 what greater than that of Ireland ; the elevation of the delta does not, 

 in any part, exceed 10 feet. According to Sir Charles Lyell, the larger 

 portion of this area consists of swamps, supporting a luxuriant 

 growth of timber, especially cypress trees, interspersed with lakes 

 formed in deserted river bends. The mass of vegetable matter is 

 increasing, for as one generation of cypress trees moulders down, 

 another rises above its remains — each generation, in its course, adding 

 to the vegetable accumulation. At the bottom is an unctuous clay, 

 which is penetrated by the cypress roots, in a manner similar to the 

 Stigmarise in the underclays of a coal-seam. Portions of these swamps 

 are occasionally covered over by sediment, brought down by the 

 river, which, when swollen, breaks through its banks, and a muddy 

 or sandy roof is formed to the carbonaceous mass, like that which is 

 found in the Coal-measures. Sections indeed show, that within a 

 period comparatively recent, beds of vegetable matter, consisting 

 chiefly of mouldering cypress trees, identical with those now growing 

 in the swamps, are overlaid by strata of clay and sand 80 feet in 

 thickness. A depression of the delta, to the extent of only a little 

 above 10 feet, would bring an area of 14,000 square miles below the 

 level of the sea ; the materials of an extensive coal-bed would be 

 covered over ; mud, sand, and lime strata would be formed, and 

 marine exuviae entombed ; and the series, after the lajise of ages, 

 would present a counterpart to the Mountain Limestone of Northum- 

 berland and Berwickshire. Nor is such a depression unlikely to 

 occur ; for changes of level are going on at present, some gradual, as 

 in Scandinavia, and others sudden, as on the coast of Chili. 



