THE DATE OF THE GLACIAL PERIOD. 595 



fill the basins, and cover the bottom with a floating of clay- 

 like mold. So rapid is the work of these minute beings, that 

 in some cases from six to ten inches of this mud is deposited 

 in one year. Some artificial basins in the large ornamental 

 parks of Europe have to be cleaned of such muddy deposits of 

 floating plants, mixed with small shells, every three or four 

 years. 



When left undisturbed this mud becomes gradually thick 

 and solid — in some cases of great thickness, affording a kind 

 of soil for the growth of marsh-plants, which root at the bot- 

 tom of the basius or swamps and send up their stems and 

 leaves to the surface of the water or above it, where their 

 substance becomes in the sunshine hard and woody. 



As these plants periodically decay, their remains, of course, 

 drop to the bottom of the water ; and each year the process is 

 repeated, with a more or less marked variation in the species of 

 the plants. After a time the basins become filled by these suc- 

 cessive accumulations of years or even centuries, and then the 

 top surface of the decayed matter, being exposed to atmos- 

 pheric action, is transformed into humus and is gradually cov- 

 ered by other kinds of plants, making meadows and forests. 



In this way many deposits of peat are buried under- 

 ground and remain unknown until discovered by diggings or 

 borings. Such are the immense peat deposits in the great 

 swamps of Virginia, the Dismal Swamps, and all along the 

 shores of the Atlantic from Norfolk to New Orleans. 



In other cases when basins of stagnant water are too 

 deep for the vegetation of aquatic plants, Nature attains the 

 same result by a different special process, namely, by the pro- 

 longed vegetation of certain kinds of floating mosses, espe- 

 cially the species known as sphagna. These floating masses 

 grow with prodigious speed, and, expanding their branches in 

 every direction over the surface of ponds or small lakes, soon 

 cover it entirely. They thus form a thin floating carpet, which, 

 as it gradually increases in thickness, serves as a solid soil 

 for another kind of vegetation, that of the rushes, the sedges, 

 and some kinds of grasses* which grow abundantly mixed with 

 the mosses, which by their water-absorbing structure furnish a 

 j>ersistent humidity sufficient for the preservation of their re- 



