IMPORTANT TYPES OF PEAT MATERIAL. 21 
Especially interesting in this connection are the marly and diato- 
maceous accumulations, which on account of their importance are 
treated here as subtypes. 
Characeous subtype. — Structureless, mainly calcareous plant re- 
mains, predominantly the deposition of calcium carbonate by green 
algae, such as the stoneworts Chara and Nitella ; usually comminuted 
granular fragments, soft, friable, and cream colored to white when 
pure, yellowish to dark rusty red according to the amount of iron 
present, grayish brown to black and granular when admixed with 
herbaceous plant remains. 
Shell-marl phases are quite common among various types of peat, 
but the shell remains from mollusks are not an important factor in 
the production of the larger or more extensive accumulations of 
marl. Inclusions, such as calcareous tufa, concretions and nodules 
caused by blue-green algse or by bacterial precipitation, and flaky in- 
crustations on semiaquatic plants, mosses, rhizomes of herbaceous 
plants, and a form of fine tubular material derived from mats of 
stonewort buried essentially intact, are occasionally found embedded 
in the macerated types of peat material. Most of the calcareous sub- 
type of plant remains is covered with herbaceous organic matter, 
but outcrops and layers between beds of peat material are frequent in 
level valleys. 
Extensive deposits of marl and of marly peat materials of other 
types occur mainly in regions underlain by rock formations contain- 
ing limestone and in areas where streams, underground and spring 
waters, or the soil material adjacent to and underneath a peat area 
are derived from calcareous drift. 
Sandy, argillaceous, ferruginous, phosphatic, and peaty marls owe 
their names to their obvious admixture. Marly phases of peat are as 
a rule granular, grayish brown to blackish gray in color, and inclined 
to be high in available nitrogen when under cultivation. To re- 
store a proper balance in the ratio of plant food constituents, they 
therefore should not be treated with nitrogenous fertilizers. Where 
the amount of lime is too high, crops tend to become chlorotic and are 
susceptible to nutritional disturbances. 
Beds of pure marl are used in the manufacture of cement, and they 
furnish lime of value as an agricultural fertilizer. 
Diatomaceous subtype. — Structureless, mainly siliceous material 
from plant remains consisting of diatomaceous shells admixed with 
a varying proportion of sponge spicules, spore cells, macerated or 
fibrous plant material, and drift debris ; light gray in color and com- 
pact but of very little weight in proportion to its air-dry bulk; per- 
vious when free from impurities ; blackish gray and plastic to sticky 
