BOG-IRON ORE. 143 



The pureness of the peat in these cases is due to the fact that the mud- 

 dy waters of the river are strained of all their sedimentary matter by 



Fig. 114.— Meal section through Dismal Swamp, a a, original soil ; b b, peat ; I, lake. 



passing through the dense jungle-growth of cane and herbage which 

 surrounds these favored spots. Thus only pure water reaches them.* 

 Similar peat-swamps are found at the mouths of the Ganges, the Kiger, 

 and other great rivers. 



Alternation of Peat with Sediments. — We have already stated (page 

 136) that a section of the delta-deposit of many great rivers, such as the 

 Mississippi, Ganges, and Po, reveals alternate layers of fresh-water and 

 marine sediments, with thin layers of vegetable mold containing 

 stumps. In some cases these layers of vegetable mold amount to con- 

 siderable thickness of turf or peat. Layers of peat two feet thick have 

 been found between layers of river-mud in the delta of the Ganges 

 (Ly ell's Principles of Geology). Similar layers have been found in 

 the delta of the Po. They are evidently submerged peat-swamps. 

 These facts are of great importance in the explanation of the accumu- 

 lation of coal. 



Drift-Timber. 



Great rivers in wooded countries always bring down in large num- 

 bers the trunks of trees torn from the soil of their banks. These trunks 

 lodging near their mouths, where the current is less swift, and accumu- 

 lating from year to year, form rafts of great extent. The great raft 

 of the Atchafalaya, which was removed in 1835 by the State of 

 Louisiana, was a mass of timber ten miles long, seven hundred feet 

 wide, and eight feet thick. It had been accumulating for more than 

 fifty years, and at the time of its removal was covered with vegetation, 

 and even with trees sixty feet high. Similar accumulations of drift- 

 wood are described as occurring in the Eed Eiver, the Mackenzie 

 Ewer, and in Slave Lake. Such rafts become finally imbedded in 

 river-mud, and undergo a slow change into lignite or imperfect coal. 

 Beds of partially- formed lignite are therefore found in sections of the 

 delta-deposit of almost all great rivers. We will use these facts in 

 speaking of the theories of the coal. 



Sectiok 2. — Bog-Iro^ Ore. 



At the bottom of peat-bogs is often found a " liar d pan " of iron- 

 ore, sometimes one to two feet thick. The same material often collects 

 in low spots, even when there is no decided bog. The manner in which 



* Lyell's Elements of Geology, fifth edition, p. 385. 



