BEITISH MOSSES. 57 



between these two kinds of peat may easily be ascertained 

 by anyone who will, as I have done, subject the two kinds 

 to the action of water. 



In Fig. 83 will be found a section of a peat bog, copied 

 from an engraving in the third Eeport of the Commissioners 

 on Irish Bogs (" Parliamentary Papers," 1813-4), and 

 exhibiting the remains of three forests anterior to the 

 vegetation growing on the surface of the bog. The history 

 of the formation will be, I believe, much as follows : 



(1) We must get a watertight bottom. In the section 

 given it is said to consist of limestone gravel, but this 

 probably had, at least in its lower part, got consolidated 

 into a pan by the infiltration of insoluble iron oxides, 

 themselves often due to decaying vegetable matter, or it 

 rested on a subsoil of stiff clay. The necessity of this 

 watertight bottom is well shown by the fact that in places 

 in the Irish bogs where a pure limestone subsoil occurs the 

 bog becomes shallow and dry. 



(2) On this limestone gravel a forest arose and flourished 

 for a considerable period, until the natural drainage of 

 the area was stopped, whether by the choking up of the 

 course of the effluent stream, or from the aggregation of 

 vegetable matter, or from the fall in the course of nature 

 of the trunks of the trees themselves. Everyone who will 

 consider how much care our rivers require in order to 

 make them flow with regularity to the sea who thinks, 

 for instance, of the works in the Thames valley, or in the 

 upper valleys of the Rhine will see how often and how 

 easily, in a country in the condition of nature, stagnant 

 waters will arise. In the morass thus formed the Sphagnum 



