498 



PRINCIPLES OF S'l'RATKiRAPHY 



matic; 3, semiterrestric, and, 4, terrestric. The needs of the plant 

 associations in mineral food also vary with the development of the 

 swamp or bog, the earliest plants deriving their food from the 

 water or the bottom on which they grow, while the succeeding 

 groups must depend more and more upon the older plant deposits 

 for their mineral nourishment, and this becomes less and less in 

 amount. We have thus a series with decreasing demand on the 

 substratum for food, and these are classed in descending order, as 

 CU-, iiieso- and oUgotraphent types, and they may be found among 

 various hygrophylous types. From the deposition of these types of 

 vegetation we get eu-, meso- and oligotrophic types of peat deposits. 



Fig. 113. Ideal section, showing tiie approximate relation (i) of the different 

 types of peat, and (2) the plant societies at Algal Lake, northern 

 ]\Iichigan. (After C. A. Davis.) The succession of plant asso- 

 ciations from without inward is: (i) Tamarack-spruce-cedar 

 swamp, with young tamarack at the inner border; (2) open 

 sedge marsh, with islands of tamarack wood; (3) swamp loose- 

 strife (Decodon verticillatum) gradually advancing lakeward, and 

 forming "stools" on which grow mosses, ferns, sedges and 

 shrubs, finally killing *the loosestrife; (4) cat-tail flags; (5) 

 potamogeton. The peat formed by these plants thickens away 

 from the lake, and is humus peat. Below this, and forming the 

 lake bottom, is a mass of sapropelitic peat, composed of green 

 algt-e, wnth diatoms, and an abundance of pollen-grains of conifers, 

 forming a structureless mass. 



the last of which normally occurs in the upper part of the stratified 

 series. 



It is, however, apparent that in the gradual closing of a lake by 

 vegetal deposits, those formed near the shore will have the largest 

 supply of mineral food, especially where affluents bearing such food 

 enter, and that the waters within the growing rivers of eutrophic 

 vegetation will become poorer and poorer in foodstuff, so that the 

 inward succession of plant associations will take on successively a 

 meso- and an oligotraphent character, and the deposits of peat in 

 such a basin, when completely filled, will be eutrophic peat around 

 the margin, mesotrophic next within, while in the center it will be 

 oligotrophic, and thus of a distinct character. Thus within the same 



