634 GEOLOGY 



Preservation of vegetable matter. In marshes, where the 

 vegetation falls into water, it usually undergoes slow change different 

 from decay suffered by vegetation which falls on dry land. The 

 preserving influence of water is seen in many ways. Posts and piles 

 set partly in water and partly above, decay just above the water- 

 level, while the portions below remain sound. It is the partial 

 preservation of organic matter in the water of marshes and very 

 shallow lakes which converts them into peat-bogs, for the peal is 

 nothing more than accumulated vegetable matter undergoing 

 those changes to which vegetable matter in water is subject. Under 

 favorable conditions, the peat of a bog may become very deep, as in 

 the Dismal Swamp and elsewhere. In and about marshes and 

 swamps, therefore, we find the conditions for the accumulation of 

 considerable thicknesses of vegetable matter, sometimes nearly 

 free from sediment, and at the same time the conditions which keep 

 it from complete decay. 



Conversion into coal. But while the vegetable matter is not 

 destroyed, it is not preserved intact. The composition of w>ud 

 and peat are illustrated by the following analyses (ash omitted), 

 though neither wood nor peat has a constant composition. 



Carbon Hydrogen Oxygen Nitrogen 



Wood 49.66 6.21 43.03 1.10 



Peat 59.50 5.50 33.00 2.00 



The relative atomic proportions of carbon, hydrogen, and oxy- 

 gen in cellulose are expressed by the formula C 72 H 120 O 60 . In the 

 air, the carbon and the hydrogen of the wood unite with the oxygen 

 of the air or the wood itself, forming carbon dioxide and water. 

 the principal products of the decay of vegetation. But HIM lei- 

 water the atmospheric oxygen is largely excluded, and the elements 

 of the wood are thought to unite with one another to a larger extent . 

 while the oxygen of the air plays but a subordinate part. One MI 

 the common products of decay under such circumstances is (II, 

 (marsh-gas), which bubbles up from swamps and escapes into the 

 atmosphere. The formation of this gas exhausts the hvdm.uen <>f 

 the organic matter four times as rapidly as the carbon. If the 

 carbon of the wood unites with the oxygen of the wood, funning 



