§ 28, 29. SUBTERRANEAN FORESTS. 67 



any other variety.* Polished sections of the compact 

 masses exhibit the peculiar structure of coniferous trees, 

 and prove that the coal was derived from a species allied to 

 the American fir. It has probably been formed by the 

 chemical changes supervening upon fir-balsam, during its 

 long immersion in the humid peat ; the circumstances 

 under which it was placed being most favourable for that 

 process to take place, by which, as we shall show hereafter 

 (see Lecture VI.), vegetable matter is converted into coal. 

 In the Chatham Islands, Dr. Dieffenbach observed a bed of 

 peat, in which were layers closely resembling coal, and 

 possessing a lustrous appearance and conchoidal fracture ; 

 while in other parts of the same layers the transition to 

 true peat was evident. 



28. Subterranean forests. — Independently of the 

 trees imbedded in peat bogs and morasses, there are also 

 found entire forests buried deeply in the soil ; the trees 

 having their roots, trunks, branches, fruits, and even leaves, 

 more or less perfectly preserved. Numerous accumulations 

 of this kind have been discovered on the coasts of England, 

 occupying low alluvial plains, that are still subject to 

 periodical inundations. f The trees are chiefly of the oak, 

 hazel, fir, birch, yew, willow, and ash ; in short, almost 

 every kind that is indigenous to this island occasionally 

 occurs. The trunks, branches, &c. are dyed throughout of 

 a deep ebony colour by iron ; and the wood is firm and 

 heavy, and occasionally fit for domestic use ; in Yorkshire, 

 timber of this kind is sometimes employed in the con- 

 struction of houses. 



29. Geological effects of the ocean. — While the 

 mountains, valleys, and plains of the interior of a country, 



* An analysis of 100 grains gave the following results : — Bitumen, 

 72; Carbon, 21; Oxide of Iron, 4; Silica, 1; Oxide of Manganese, 2; 

 = 100. 



f See Illustrations of the Geology of Sussex, p. 288. 

 f 2 



