191 1-] STEVENSON— FORMATION OF COAL BEDS. 597 



but also majestic trees, such as cypress and gum. which, as well as 

 the less imposing juniper, yield wood of great importance to the 

 artificer. 



The inland swamps of the northern states dift'er in many wavs 

 from the coastal swamps. They occur along river borders or in 

 lakelet areas of the drift-covered region. In great part, the former 

 are "wet woods'" covered more or less deeply with water during 

 several months of each year, but they show considerable stretches 

 of true swamp. The swamps and marshes of the drift region are 

 less extensive, but they afford better opportunity for studies bearing 

 on the mode of accumulation. They have been investigated bv C. A. 

 Davis, H. Ries, N. S. Shaler and others, but the most comprehensive 

 and most recent description is by Davis. 



Davis^^- notes that very few highly organized plants can grow 

 wholly submerged in water, and those are mostly endogens ; lo feet 

 of depth seems to be the limit, although Potamogeton has been found 

 rooted in 23 feet : other types, low forms such as Cliara and the 

 floating algse are indift'erent. Some plants, burweeds, arrowheads, 

 reed grass, pickerel weed and water lilies can grow when partially 

 submerged ; while some land plants, shrubs and trees can endure 

 long exposure to water about the roots. The surface growth on 

 swamps is important. Elm and black ash swamps are of common 

 occurrence and have, besides those plants, tamarack, spruce, willows, 

 alders, with various heaths and mosses. They do not always show 

 much peat, but what there is is well decomposed and is apt to contain 

 much mineral matter. The greatest thickness of peat in these 

 swamps is reported to be 10 feet. Tamarack { Lari.v lariciua) and 

 white cedar (Chanuccy parts tliyoidcs) indicate the presence of peat, 

 the latter growing densely on the surface of a deposit, 20 feet thick. 

 Spruces {Picca mariana and P. brcvifoUa) also grow on thick peat; 

 willows, poplar and alders grow on the thickest peat and in wet 

 places; but the mosses, Hypmim and Sphagiiuin, grow onlv in 

 advanced swamps. 



^ C. A. Davis, " Peat, Essays on its Origin, Uses and Distribution in 

 Michigan," Rep. Mich. Geol. Survey for 1906, pp. 121-125, 128-134, 136-141, 

 153. 154, 157-159, 160-166. 203, 204, 208, 213, 269, 275, 279, 291. 



195 



