240 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



the rocks. Even the rootlets of grass and other vegetation 

 bore their way through sub-soil, and thus prepare an easier 

 path for the infiltration of surface water and its combination 

 with the organic acids as it proceeds on its subterranean 

 passage. While, therefore, a thick covering of vegetation 

 helps to protect the ground from sudden landslips and rapid 

 surface denudation, and has a beneficent influence upon 

 climate, the decay of vegetation slowly and surely rots any 

 mineral matter within reach of the powerful humus acids. 



Peat-mosses occupy wide areas in the Temperate and Arctic 

 zones, and have been frequently made the subject of scientific 

 researches. In 1810, Rennie published his work, Essays on 

 Peat-Moss, an able treatise on the Scottish peat-mosses; and 

 the nature and origin of peat-deposits were afterwards eluci- 

 dated in handbooks by Dau (1823) and Wiegmann (1837). 

 What Rennie achieved for the Scottish peat-mosses, was done 

 for the Danish and North German peat-deposits by Steenstrup 

 (1841) and Griesebach (1845). These authors defined for the 

 first time the differences between Sphagnum mosses char- 

 acteristic of marshes on mountain-slopes and valleys; low- 

 lying or lacustrine growths and deposits of peat rich in rushes 

 and sedges; and forest-peat or swamps. A typical example of 

 a forest moss is the " Dismal Swamp " in Virginia, which Lyell 

 described in 1841, and Lesquereux afterwards examined in 

 more detail. 



Modern deep-sea researches have discovered a few instances 

 of marine peat; and according to the new investigations of 

 Eugene Bertrand, isolated coal-beds occur which have been 

 mainly formed by sea-weeds, for example the " Boghead " coal, 

 near Autun, and the "Kerosene" in Australia. The low 

 coasts, estuaries, and river-mouths in tropical lands are fre- 

 quently fringed by mangrove-trees whose withered roots and 

 fallen radicles form coaly deposits on the sea-floor, mixed with 

 a large proportion of the finer coastal detritus. In a similar 

 way, drift-wood may accumulate in large rivers, and by the 

 process of subaqueous decay may be converted into lignite, 

 or a substance of the nature of brown-coal. Lyell's descrip- 

 tion of the "rafts" of the Mississippi will be familiar to most 

 readers. 



Fossil brown-coal may be compared with these recent forma- 

 tions. The origin of brown-coal from plant-decay has never 

 been questioned. A valuable monograph on brown-coal, describ- 



