140 ORGANIC AGENCIES. 



mation of these must be a phenomenon somewhat different from those 

 local oscillations which alone have been the subject of direct observa- 

 tion. Such general changes can only be the result of gradual unequal 

 contraction of the whole earth consequent upon its secular cooling. 

 The further discussion of this theory, however, belongs properly to the 

 second part of this work. 



CHAPTER IV. 



ORGANIC AGENCIES. 



As agents modifying the crust of the earth, organisms are, per- 

 haps, inferior to the agents already mentioned (although the immense 

 thickness and extent of limestone strata are a monument of their power 

 in this respect) ; nevertheless, they are peculiarly interesting to the 

 geologist as delicate indicators of climate, and recorders of the events 

 of the earth's history. We will take up the subject of their agency 

 under three heads, each having a separate application in interpreting 

 the structure and history of the earth, viz. : 1. Vegetable Accumula- 

 tions, to account for coal and bitumen ; 2. Bog-Iron Ore, to account 

 for iron-ores inclosed in the strata ; 3. Lime Accumulations, to account 

 for limestones, etc. 



Section 1. — Vegetable Accumulations. 



Peat-Bogs and Peat-Swanks. 



Description. — In humid climates, in certain places, badly drained 

 and overgrown with moss and shrubs, a black carbonaceous mud accu- 

 mulates often to great depths. This substance is called peat or turf, 

 and such localities peat-bogs. The thick mass of vegetation which 

 covers their surface, with its interlaced roots often forms a crust upon 

 which a precarious footing may be found, but beneath this is a tremu- 

 lous, semi-fluid quagmire, sometimes twenty to forty feet deep, in 

 which men and animals, venturing in search of food, are often lost. 

 These bogs are most numerous in northern climates. One tenth of the 

 whole surface of Ireland, and large portions of Scotland, England, and 

 France, are covered with peat. The bog of the Shannon River is fifty 

 miles long and three miles wide ; that of the Loire in France is 150 

 miles in circumference. Extensive bogs exist also in the northern por- 

 tions of our own country. The amount of peat in Massachusetts alone 

 has been estimated at more than 15,000,000,000 cubic feet (Dana). In 

 California, an imperfect peat covers large areas about the mouth of the 

 San Joaquin River and elsewhere (tule-lands). In more southern cli- 



