Page:The American Cyclopædia (1879) Volume IV.djvu/751

Rh a sandy bottom without an intermediate clay bed. This happens especially when shallow basins of water are closed by sand deposits, and thus totally separated and guarded against the invasion of outside water and the muddy materials which might be brought upon it. These basins have a permanent level, and thus the work may proceed without interruption. The peat formed in that way rests immediately upon a bottom of sand, as coal beds are found sometimes also upon sandstone. It happens, however, that in some deeper part of the same basins the subaquatic vegetation is established, and its decomposition forms beds of clay or a clay bottom whereupon the old coal plants take root at a later period. The same bed may therefore at different places rest upon clay or upon sand. This fact also is remarked in coal beds of some extent whose bottom is at intervals either clay or sandstone. It also frequently happens, especially in small lakes and bayous, that the peat vegetation begins at the surface with floating masses, which gradually invade the whole space, covering it with a vegetable carpet. By successive annual growth and deposits this groundwork becomes thick and more solid; shrubs and even trees take root and grow upon it, till the mass becomes too heavy, is split or torn asunder, and sinks to the bottom. Other series of vegetation may begin again in the same way and be successively heaped upon one another, peat beds and forests, till the basins are filled. From some of these filled lakes now discovered under thick strata of humus, peat and wood, and even large trees which could be used for building, have been dug out to a thickness or a depth of 75 ft. or more. In New Jersey a considerable business has been done in fishing out of peat bogs the buried cedar timber. At other localities, as in the old bayous at the mouth of the Mississippi, floating islands, strong enough to support the construction of railways, have been formed in the same manner. Their nature is recognized by the vacillation and unsteadiness of the ground, which undulates under the pressure of a heavy weight like that of a railway train. At other places, as in the Dismal swamp, the formation is mixed. A bed of peat 15 ft. thick is formed over the swamps around Drummond lake, by the debris of large trees and of an impenetrable grove of canes, whose roots penetrate deeply into thick layers of sphagnum. It is there extremely difficult to reach the borders of the lake. They are mere floating moors, sinking under the weight and formerly extending over and covering the whole surface of the lake. This is proved by a layer of trees strewn at the bottom of the lake, and the process of nature may be seen still in activity along the borders, where large trees (bald cypress), sunk into the water, perhaps 8 ft. deep, are there slowly decaying in a standing position. Their trunks are already generally hollow, only the bark and a thin crust of wood being left; the water enters them of course, and fills them with

the debris floating upon the surface, especially cones and leaves. In time these decayed trunks fall to the bottom and are there imbedded either for slow decomposition to peat, or if covered with mud petrified by the softening of their tissue and the impregnation of mineral matters. In that way the hollow trunks discovered in the coal beds of England and Canada have been petrified; their bark only is preserved, and they are filled with cones, leaves, insects, shells, &c. These fossil trees were looked upon for a long time as an inexplicable wonder.—Various circumstances, as local depressions which bear water upon the surface of the emerged peat bogs and destroy their vegetation, stop the growth of the peat and change the surface into mud swamps. This is especially caused by alternation of level, or by the ground being covered with water at one time of the year and dry at another. Then the decomposition of the plants is rapid and entire, and its result is a kind of mud increased by the introduction of foreign materials brought by water. Leaves and vegetable debris are often imbedded in this matter and thus preserved by petrifaction. This represents the formation of the laminated clay shale which generally covers the coal beds as a roof, and contains in some localities vegetable remains in a beautiful state of preservation. Of course, these changes of level were formerly, especially at the carboniferous period, more frequent than now; for then the land was mostly of insecure formation, half floating, and along the sea immense tracts of land were engulfed, and so deeply sunk that their surface was covered with sand by the currents, or even at a greater depth invaded by the animals building and forming limestone deposits, shells, madrepores, corals, &c. In this way the formation of the sandstone and limestone which in some places overlie the coal beds without intermediate shale is easily understood. Cases of a similar nature are recorded from the peat bogs of our time. It has been said already that in Holland old peat bogs have been discovered by borings, at a depth of from 60 to 100 ft. below the surface, placed upon one another and separated by clay deposits of 30 ft. or more. The great peat bogs between the lakes along the foot of the Jura mountains in Switzerland sink in places under the sands of the lakes and are covered by gravel. At other localities the peat deposits of a thickness of 10 ft. are cut in the middle by a layer of coarse sand varying from 6 to 12 in. in thickness. We see therefore in the present formation of the peat bogs a counterpart of that of the coal, represented in all its details, though in a reduced proportion. Not a single case has been recorded in regard to the formation of coal which cannot find its counterpart and its explanation in some of the phenomena attending the present formation of peat. In the countries where peat banks are exposed and worked to their base by drainage, and where the nature