LARAMIE, OR TRANSITION EPOCH. 495 



by which the great interior Cretaceous sea, which previously divided 

 America into two continents, was abolished, and the continent became 

 one. At the same time the Wahsatch and Uintah Mountains were 

 principally formed, and the eastern Rocky Mountain range greatly 

 elevated. If the end of the Jurassic was pre-eminently a time of 

 mountain-making (Sierra revolution), the end of the Cretaceous was 

 pre-eminently a time of continent-making. The disturbance, as usual 

 with those which close an era, was probably to some extent oscillatory — 

 i. e., the continent was probably higher and cooler during the latter 

 part of the Cretaceous than during the subsequent Eocene. The 

 change of physical geography was enormous, and the change of climate 

 was doubtless correspondingly great. We ought to be prepared, there- 

 fore, to find, with the opening of the next era, a very great change in 

 the organisms. 



Laramie, or Transition Epoch. 



In the schedule on page 474, we have indicated a transition epoch 

 called the Laramie. There has been much controversy about the true 

 position of these strata. Some have put them in the Tertiary, some in 

 the Cretaceous, and some have regarded them as completely transitional 

 between the two ; while still others would solve the difficulty by assigning 

 the lower part to the Cretaceous and the upper part to the Tertiary. 

 Stratigraphically the Laramie is continuous with the Cretaceous below, 

 and in some places also with the Tertiary above ; so that the Creta- 

 ceous of the West in some places gradates through the Laramie into the 

 Tertiary without break. This is especially true in California, where 

 the Upper Cretaceous gradates completely and without the least break 

 through the Tejon group into the Tertiary. The difficulty of drawing 

 the line of separation on paleontological grounds is equally great. 

 The plants are decidedly Tertiary in general aspect, but the animals, 

 especially the land-animals, are as decidedly Cretaceous; the shells 

 meanwhile passing from the marine through brackish- water into fresh- 

 water forms. Cretaceous Dinosaurs still linger, but Tertiary types of 

 Plants have already taken possession. Many palseobotanists claim it 

 for Tertiary. Nearly all palaeozoologists put it in the Cretaceous. 

 There is little doubt that it is really transitional, although probably 

 more closely allied with the Cretaceous. 



The explanation of these facts is obvious : We have seen that at the 

 end of the Cretaceous the great interior Cretaceous sea was abolished 

 by elevation, and its place (as we shall see hereafter) was partly occu- 

 pied by great fresh- water lakes. Now, this change took place somewhat 

 gradually, the oceanic condition passing into the lake-condition 

 through an intermediate brackish-water condition of isolated seas, the 

 sedimentation going on all the time. While oceanic conditions pre- 



