142 
AGE CHARACTERISTICS OF COALS 
in mineral content, showing variations in conditions during for¬ 
mation. Bones of mammals, shells of fresh-water mollusks, 
and remains of insects are of common occurrence. Peat deposits 
have yielded the best specimens of Pleistocene mammalia; domestic 
cattle are often mired in swamps; and whole troops of armed men 
have perished in Scottish swamps during flight after defeat in 
battle. 
Composition of peat depends ordinarily upon its age; that at the 
bottom of the deposit not only approaches complete disintergration, 
so that to the unaided eye it shows no trace of organic structure, 
but it also is far advanced in carbon-enrichment. Yet peat from 
neighboring localities where conditions seem to have been similar, 
may show dissimilarity"in composition. One finds strange con¬ 
trasts even in benches of a single deposit, for some may be far 
advanced, while others consist of almost unchanged plants. , 
Tertic coals are known as lignite. The passage from lignite, 
or brown coal, is extremely gradual. In Europe generally the 
complex group designated as brown coal is always sharply set off 
from the Paleozoic, or “Stone” coals, so that, since Mesozoic coals 
are comparatively inconsequential, the effort there is to ascertain 
why brown coal and stone coal are so unlike, and to discover 
reasons why the former could not be converted into the latter. 
In North America the conditions are entirely different; the coals 
of all types from wood-like lignite to bituminous, and even to an¬ 
thracite, occur at times within a single district, in a single bed, 
or even within the limits of a singles estate. 
The passage from one type to another is so gradual that chem¬ 
ists and geologists of North America have labored hard and long 
to discover some means for distinguishing them. The problem 
is no longer one of mere abstract or scientific interest; it of the 
utmost practical importance, since within vast areas the only source 
of supply is in the Tertic and Cretacic formations. The effort is 
to determine distinctions which will be available for both the 
seller and the purchaser of fuel. Coal is found in all portions of 
the Tertic succession. 
Tertic coals are notably circumscribed, their areas varying from 
a few square yards to several hundreds of square miles, in some 
instances apparently even to 2000 square miles. The expanse 
being limited by the nature of the topography as the deposits ap¬ 
pear, for the most part, to have accumulated in shallow lakes or 
