146 
AGE CHARACTERISTICS OF COALS 
laid down at a notable distance from the source of the materials 
and under a practically constant cover of water, coal is absent. 
The relations are sufficiently clear in the Late Cretacic succes¬ 
sion of Europe. The immense area of Cretacic formations in the 
United States and Canada affords ample opportunities for com¬ 
parisons. Each formation, with the possible exception of the 
Niobrara chalks, is coal-bearing. The chief source of detritus 
was at the west, though important contributions were received 
from the southern border, which probably lay in northern Mexico 
not far from the international boundary. 
In form the Cretacic coal deposits are all distinctly lenses. No 
statement to this effect is found in any of the older works on the 
fossil fuels, since nearly all writers prior to less than 25 years 
ago, held in somewhat hazy way that coal seams were of continu¬ 
ous deposits. Comparation of sections in all fields proves that this 
conception was erroneous. The Wealden coals of Hanover are 
local, present in one section, absent in others, and in all cases they 
have small areal extent. There is a rather persistent coal horizon 
at the base which seems to be made up of overlapping lenses. 
The lower Quander district has only nests of coal which occasion¬ 
ally become workable; the Hungarian coals are well defined 
lenses, as are also those of Queensland; and the detailed studies 
in New Zealand have proved lens-form in the great coal seams 
of that region. 
During later years the same condition in North America is so 
marked that it has been noted by the great majority of observers. 
Occasionally a seam has an area so extensive that the describer is 
unwilling to commit himself as to the form. But it must be re¬ 
membered that, even though the lenses have areas of hundreds or 
thousands of square miles, the general features are the same as 
those of the smaller lenses united by transgression to form the 
larger one. 
Laramie coals are disposed in lenses usually small and thin with¬ 
in the United States. The great coal-bed of the Saskatchewan 
region, in Alberta, becomes merely a thin deposit of carbonaceous 
shale in its southern extension. The Fox Hills seams are lenses 
usually thin and impure, but locally important and workable over 
considerable areas. This feature is particularly noteworthy in all 
districts along the eastern base of the Front Ranges in New 
4 
