24 BULLETIN 802, U. S. DEPARTMENT OF AGRICULTURE. 
association and the rise of the water table is favorable for their 
growth, they become buried as extensive or fragmentary layers, essen- 
tially intact and fibrous or partially disintegrated. On account of 
the slight shrinkage of the plant remains, the respective layers which 
are "built up " in this manner may constitute several feet of a deposit, 
but further increases in thickness practically cease unless there is 
considerable disintegration and shrinkage in the material or a rise in 
the ground- water level from any cause. 
When an accumulation of structureless or macerated type of peal 
material formed in large part by aquatic vegetation units preceded, 
and reached nearly to the water level, peat deposits of this class show 
as a rule a fibrous matted layer, more or less sharply differentiated 
in structure from the peat material below it. By the constantly ad- 
vancing growth of semiaquatic and marsh plants the fibrous mate- 
rials may gradually extend over the shoaling water, sink as their 
thickness increases, and thus give rise to filled-in deposits of peat. 
Water pockets are not uncommon as delimiting strata. 
This has an important bearing upon drainage, tillage, and other 
operations, and upon the possible uses of the land. In planning 
drainage improvements it is well to remember that deposits not too 
well drained are safer than those overdrained and that in this 
respect the distance between ditches and tiles is usually more 
effective and important than their depth. For areas of greater 
depths of peat or with poorly disintegrated grades of fibrous peat 
material it is necessary to anticipate a gradual but certain sub- 
sidence to as much as one-fourth of the original thickness. The 
decrease in the elevation of the surface should be met, therefore, by 
a provision for deepening the gravity outlet or for drainage by means 
of pumps. 
The marsh types of peat are among the most profitable for agri- 
cultural purposes, though several of them indicate in some instances 
the presence of salts which in sufficient quantity may injure certain 
cultivated crops. The nature of the mineral subsoil, whether clay 
or sand, may considerably affect the agricultural value and need 
for fertilizers, the former responding, it seems, more readily to phos- 
phates, the latter requiring phosphatic and potash manures, and to 
some extent lime. Marsh or fen types of peat are easily cleared and 
brought under cultivation, and they are suitable for a variety of 
crops if carefully drained or if the drainage channels are provided 
with check gates. Some of the types when overdrained are apt to 
become brittle and easily break down to light-colored "mull " or 
granular peat dust. This is especially characteristic of fibrous 
sedge and of brown-moss (Hypnum) types of peat. Moreover, 
marked differences may arise from the preponderance of semiaquatic 
