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evolution of the organisms went steadily on. The smaller divisions of 
time are marked by the less important changes that the animals and plants 
suffered; while the primary divisions are signalized by the profounder 
modifications of the living beings. These primary divisions are often in- 
dicated by such phrases as the age of mollusks, the age of fishes, and the 
age of mammals. As there were no universal cataclysms that character- 
ized the terminations of the ages and the eras, so there were no sudden 
changes in the nature of the animals and the plants. The boundaries be- 
tween the successive ages and the successive eras must therefore be more 
or less arbitrarily drawn. If one era is characterized by numerous 
powerful reptiles and a few inconspicuous mammals, while the next er: 
presents mammals as the dominant animals, the reptiles as decadent, we 
must draw the line to suit our convenience and to express best the facts; 
but in the end it will be drawn more or less arbitrarily. 
To appreciate the futility of seeking for great unconformities between 
the rock systems one has only to consider the relations of the Upper 
Cretaceous to the Tertiary in Hurope. Lyell regarded the Thanet sands 
and certain equivalents in France and Belgium as the base of the Hocene. 
Between this and the Upper Cretaceous there appeared to be one of the 
profoundest breaks in geological history. Lyell says that the interval be- 
tween the Upper Cretaceous and the Hocene must have been greater than 
that between the Hocene and the present. More recent investigations 
have shown that even in the north of Hurope there are deposits of no great 
thickness that partly fill the gap between the two systems; while it is al- 
most filled in the south of that country. 
The conclusion applicable to the question being considered which I 
reach is that the magnitude of the break below the Arapahoe formation 
in the Denver basin has little or nothing to do with the determination of 
the boundary line between the Mesozoic and the Cenozoic. The position 
of this line is to be settled through the study of the organic remains found 
below and above the unconformity and the comparison of these with the 
fossils found at corresponding levels in regions geologically better under- 
stood. If the ensemble of the organisms found in the Arapahoe, the Den- 
ver, the Lance Creek and the Hell Creek beds, is essentially of Upper Cre- 
taceous nature, on comparison with accepted standards, those beds belong 
to the Mesozoic, not to the Cenozoic, notwithstanding the great uncon- 
formity. 
