280 
inhabitants. For sufficient reasons we divide this history into principal 
and subordinate portions, each having its own characteristics. Hach con- 
tinent has had its own course of development, physical and biological, this 
course sometimes agreeing only in a general way with that of other conti- 
nents, being perhaps ahead of them or behind them, possibly sometimes 
only different. In order to compare and describe contemporary conditions 
in different lands there must be a few fixed dates from which to reckon 
the march of time and progress. These dates are found in the limits be- 
tween the primary divisions, as that between the Silurian and the De- 
vyonian or that between the Cretaceous and the Tertiary. In a similar 
way we orient the history of even a savage people with reference to such 
dates as the founding of Rome and the birth of Christ. 
3. THe PRIMARY DIVISIONS OF GEOLOGICAL TIME ARE Nor USuALLY INDI- 
CATED BY GREAT UNCONFORMITIES. 
Inasmuch as those geologists and paleontologists who favor the refer- 
ence of the Arapahoe and the Denver beds of Colorado, the Lance Creek 
beds of Wyoming and the Hell Creek beds of Montana to the Eocene, give 
as their principal reason therefor the existence.of a great unconformity 
between the Arapahoe and the formation immediately below it, while there 
appears to be no similar unconformity below the Fort Union, it may be 
worth while to examine the adequacy of the reason. I believe that it is 
fallacious. 
It is possible that, as Chamberlin and Salisbury suggest in their gen- 
eral work on geology (Geology, iii, p. 192), there is a natural basis for the 
larger divisions of geological history: that this basis is to be found in 
the profounder changes in the earth’s crust; and that this basis is of 
world-wide application. This suggestion may be accepted as valuable 
without its arousing the expectation that a great stratigraphical break 
will be discovered everywhere between each great rock system and its 
predecessor and its successor. As a inatter of fact, as geological history 
is now understood and now divided, such breaks are not commonly found. 
I will quote from Geikie’s Text-book of Geology, ed. 4, 1905, p. 1081: 
Though no geologist now admits the abrupt lines of division which 
were at one time believed to mark off the limits of geological systems and 
to bear witness to the great terrestial revolutions by which these systems 
were supposed to have been terminated, nevertheless the influence of the 
ideas which gave life to these banished beliefs is by no means extinct. 
