616 DYNAMICAL GEOLOGY. 



thus filled in with limestone, bears probably evidence in itself that it 

 has been formed, not in the deep ocean, but within the reach of current 

 or wave action. 



The kinds of limestone made through the agency of life include 

 soft marl or calcareous earth, chalk, compact limestone, sometimes 

 oolitic or concretionary (p. 63), of white, gray, bluish, blackish, and 

 other colors, — the dark colors mostly due to the presence of carbon, 

 from animal or vegetable decomposition. 



4. Examples of the Formation of Strata through the Agency of Life. 

 1. Peat Formations. 



Peat is an accumulation of half-decomposed vegetable matter, 

 formed in wet or swampy places. In temperate climates, it is due 

 mainly to the growth of spongy Mosses, of the genus Sphagnum. 

 This plant forms a loose turf, and has the property of dying at the 

 extremities of the roots, as it increases above ; and it thus may gradu- 

 ally form a bed of great thickness. Moreover, it is very absorbent of 

 moisture. In some limestone regions, the Sphagnous mosses are re- 

 placed by species of Hypnum, as in Iowa. The roots and leaves of 

 other plants, or their branches and stumps, and any other vegetation 

 present, may contribute to the accumulating bed. The carcasses and 

 excrements of animals at times become included. Dust may also be 

 blown over the marsh by the winds. 



In wet parts of Alpine regions, there are various flowering plants 

 which grow in the form of a close turf, and give rise to beds of peat, 

 like the moss. In Fuegia, although not south of the parallel of 56°, 

 there are large marshes of such Alpine plants, the mean temperature 

 being about 40° F. On the Chatham Islands (380 miles east of New 

 Zealand), peat thus formed has a depth of fifty feet. 



The dead and wet vegetable mass slowly undergoes a change, be- 

 coming an imperfect coal, of a brownish-black color, loose in texture, 

 and often friable, although commonly penetrated with rootlets. In 

 the change, the woody fibre loses a part of its gases ; but, unlike coal, 

 it still contains usually twenty-five to thirty-three per cent, of oxygen. 

 Occasionally, it is nearly a true coal. 



Peat-beds cover large surfaces of some countries, and occasionally 

 have a thickness of forty or fifty feet. One-tenth of Ireland is covered 

 by them ; and one of the " mosses " of the Shannon is stated to be 

 fifty miles long and two or three broad. A marsh near the mouth of 

 the Loire is described by Blavier as more than fifty leagues in circum- 

 ference. Over many parts of New England and other portions of 

 North America, there are extensive beds. The amount hi Massa- 



