286 PAST CLIMATES AND CLIMAXES. 



great one — has even been regarded as one of the greatest breaks recorded in 

 the strata of the continent. Locally, however, the association of the Laramie 

 and the Eocene is so intimate that agreement concerning the reference of 

 certain beds, and even thick formations, has not been reached. Within what 

 has often been called the Laramie series there are local unconformities. But 

 there appears to be one unconformity which is neither slight nor local. The 

 beds above it and below it have sometimes been known as the Upper and Lower 

 Laramie respectively. In Colorado the beds above the great unconformity 

 have also been called post-Laramie, and have sometimes been classed with 

 the Cretaceous, and sometimes with the Tertiary. They include the Arapahoe 

 (below) and Denver formations. 



"Traced eastward, the Denver beds pass beneath Miocene beds. Strati- 

 graphically, therefore, there is no reason why the Arapahoe and Denver forma- 

 tions should not be referred to the Eocene. The fossil plants of the Denver 

 formation, of which something like 150 species have been identified, are con- 

 sistent with this interpretation. But few species are common to the Denver 

 and Laramie of Colorado, while an equal proportion are common to the Denver 

 and the Eocene of other localities. The meager Arapahoe flora is more closely 

 allied with the Denver flora above than with the Laramie flora below. The 

 invertebrate fauna of the Denver beds is little known, and the identified species 

 are common to both Laramie and Eocene. The vertebrate fauna has distinct 

 Mesozoic affinities, and has been the chief reliance in classing the Arapahoe and 

 Denver formations with the Laramie. If the presence of saurian fossils 

 demonstrates the Cretaceous age of the beds containing them, the Arapahoe 

 and Denver beds are Cretaceous; but every other consideration seems to 

 point rather to their reference to the Early Tertiary. After the deposition of 

 the Laramie below, and before the deposition of the Arapahoe and Denver 

 beds, there were great orographic changes, a long inteiTral of erosion, and the 

 initiation of the protracted period of vulcanism which marked the close of the 

 Mesozoic. These physical changes were accompanied by marked changes 

 in vegetation and these changes had been accomplished before the deposition 

 of the Denver beds. The great physical changes which inaugurated the 

 changes in life appear to have taken place before the Arapahoe formation was 

 deposited. Their effects had distinctly modified plant life by the time the 

 Denver beds were deposited, but they appear to have had less effect on the 

 vertebrate life of the west, perhaps because conditions were not yet favorable 

 for the incoming of the mammalian life from the regions where it originated." 



Evidence from the method of sequence. — It is suflSciently obvious that the 

 diAdsions of the earth's history must be based upon changes in the earth's 

 crust, its atmosphere and biosphere. Geological evidence of such changes 

 abounds in all regions, but the evidence itself is secondary in importance to 

 its sequence. The latter is determined wholly by stratigraphy, or the order 

 of strata and formations, and by fossil horizons. While all the direct evidence 

 must be derived from stratigraphy and fossils, much of this must be inter- 

 preted in the light of existing relations. It was inevitable that the much greater 

 abundance of stratigraphic and paleozoologic materials should have resulted 

 in their being used as evidence to the practical exclusion of plant Ufe. Such 

 usage does not seem wholly warranted by the facts, and it is especially out of 

 harmony with the basic sequence of cause and effect. As has been already 

 indicated, major deformations of the earth's crust result primarily in changes 

 in land and water relations, and these produce changes of climate. The latter 

 effect changes in the dominance or distribution of vegetations, and the modi- 



