52 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



delta region of the Mississippi is subject to frequent floodings, but, 

 in a great part of the area, the dense " cane brakes " act as filters, 

 so that the water is freed from its load of silt and continuity of 

 swamp growth is uninterrupted. 



There are serious interruptions in the growth, which are not due 

 to flooding or to merely local variations in the water-level but rather 

 to widespread changes in conditions. Benches in thick deposits 

 frequently appear to represent cycles of deposition and these often 

 are separated by thin partings of exceedingly fine mineral matter, 

 containing more or less of fibrous material resembling the mineral 

 charcoal of ordinary coal. Such partings were explained long ago 

 by Lesquereux as due to a period of dryness, when the peat ceased 

 to grow and the surface was destroyed by oxidation to a greater or 

 less extent. The period of exposure may be brief or it may be 

 long continued. A. Geikie and Lewis have made clear that peat 

 forms now in only exceptional localities within Scotland, as the 

 climate has become less moist; and Skertchly asserts that peat is 

 no longer forming in the Fenland of England, save in a dark narrow 

 valley of Suffolk. The peat is wasting in those areas. Similar 

 statements come from other parts of northern Europe. Leaving 

 out of consideration Taxodhim, Nyssa, certain palms and the trees 

 of the Kampar areas, it is certain that most of the trees growing on 

 peat do not thrive when the material is very wet. Some, it is true, 

 show a notable degree of adaptation. C. A. Davis saw in Michigan 

 a birch in healthy condition, though its roots had been covered with 

 water during more than a year; and there are other types which 

 do well if only the water cover be absent during a considerable part 

 of growing season : the tamarack at times takes root far out on the 

 floating bog, but it grows slowly as do other trees which accompany 

 it. One may learn much respecting changing conditions during the 

 growth of a peat deposit by noting the distribution of trees. 



Zincken"*'' cites Hartig as saying that in the " rothe Bruche '' on 

 the Harz there occur in the lowest 5 feet of the 39 to 40 feet thick 

 Hochmoor, firs with stems 18 inches thick. Higher, is a layer with 

 large pines, on which is another with smaller plants of the same 



'**' C. Zincken, "Die Physiographic der Braunkohle," Hannover. 1867, p. 38. 



