76 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



ditions could exist on great plains ; lake deposits are too small to be 

 important in this connection. 



The Floor or Mur of Peat Deposits. — In regions where cal- 

 careous matter abounds, the floor of peat originating in ponds or 

 lakes is apt to be the familiar lake marl, formed by Chara and 

 mollusks, and containing other freshwater forms. Where calcareous 

 matter is lacking, fine clay is the usual floor. Marl and clay are 

 almost impervious ; the impression has prevailed that peat grows 

 only on a floor impervious to water. 



But marshes and bog deposits may originate on rock of any sort, 

 which is free from constituents injurious to plant life. The great 

 Okefinokee Swamp of Georgia and the much greater Dismal Swamp 

 of Virginia and North Carolina rest, in great part, on sand and in 

 each the peat is thick. The buried peat of the Holland-Belgium- 

 France area has mostly a floor of blue clay, though in many places 

 it rests on sand. Typical freshwater peat may overlie marine sands, 

 clays or limestones. The Carse land peats of Scotland, according 

 to J. Geikie, have as the floor marine sands or marine clays ; Mog- 

 gridge found a similar condition in the Swansea excavations. 

 Rohhumus or Trockentorf, so familiar in our forests, grows on bare 

 rock ; even granite may be the floor. 



Davis"^ saw " climbing bogs " in northern Michigan, which had 

 grown on smooth glacier polished granite. One, on an isolated rock 

 hill, showed Sphagnum in spots, evidently thrifty and making a good 

 growth, but most of the surface was covered with reindeer lichen, 

 both in the open and under trees and shrubs. The peaty cover is 

 thin and fibrous, with little moisture, but this supports the usual 

 trees and shrubs, conifers with white birch and mountain ash as well 

 as some heaths. A small island, with rounded glaciated surface and 

 embracing about 3 acres, rises about 30 feet above Bubbling lake. 

 The peat covering it is usually thin, about one foot but occasionally 

 reaching 3 feet. It is coarse, spongy and brown, contains tree 

 trunks, not thoroughly rotted, along with abundant partially de- 

 cayed roots and stems of plants. When this locality was examined, 

 the peat was so dry as to burn. The flora consists of the conifer- 



^* C. A. Davis, " Peat," 1907, pp. 264-269. 



