ORGANIC. 263 



or silicify whatever organism comes in their way. To this 

 class of deposits "belong also all superficial accumulations of 

 saline substances (common salt, soda, nitrates of soda and 

 potash, borax, &c.), sublimations of sulphur, exudations of 

 bitumen and asphalt, and indeed all formations arising more 

 immediately from chemical action and reaction. Compared 

 with mechanical accumulations, they are usually of limited 

 dimensions, but they are often of curious interest, and their 

 study throws a flood of light on many of the older pheno- 

 mena of the rocky crust. 



Among Recent Formations those of Organic origin that 

 is, arising from the growth and decay of plants and animals 

 are, perhaps, the most interesting and instructive. There 

 is no phenomenon more familiar to British readers than 

 the peat-mosses which occupy considerable tracts in every 

 part of the country, and which, before the extension of 

 modern agriculture, made their appearance over still wider 

 areas. These peat-mosses are entirely of vegetable growth 

 mosses, aquatic plants, heath, and fallen forests ; and 

 imbed isolated trunks of oak, pine, and birch, as well as the 

 remains of man, oxen, deer, and other animals that have 

 been swamped in their boggier portions. These accumula- 

 tions are of all ages, from the growth of the current year 

 back to the very close of the glacial epoch ; and their ex- 

 tent and thickness depend entirely upon the nature of the 

 area (its flatness, moisture, and shelter) in which they occur. 

 We have witnessed a depth of thirty feet reposing on blue 

 lake-silt which enclosed the remains of the extinct Irish 

 deer; while in another district similar silts have been found 

 beneath a peat-covering of less than a third of that thickness. 

 We have seen Eoman remains of the time of Agricola found 

 at a depth of twelve feet ; while in other tracts we have ex- 

 amined the hewn stumps of trees, the plank-roads, the lost 



