VEGETATION ERAS. 285 



Texas, no species of marine life is known to have lived over the time-interval 

 recorded by the unconformity between the two systems. On stratigraphic 

 grounds, the distinctness of the Lower Cretaceous from the Upper in North 

 America is such as to warrant their recognition as separate systems. Thick- 

 nesses of strata afford no basis for the separation of systems, yet it may be 

 noted that though the average thickness of the Comanchean system is not so 

 great as the average thickness of the formations of most Paleozoic periods, yet 

 its maximum known thickness (26,000 feet in California, measured by the 

 customary method) is greater than that which any Paleozoic system is known 

 to possess at any point in America. (124r-125) 



"At the opening of the (Upper) Cretaceous in America, the angiosperms 

 were in marked dominance, and during the period genera now living became 

 more and more abundant, giving to the whole a distinctly modern aspect. 

 It is worthy of remark here that the Cretaceous revolution in vegetation was 

 not only great as a phytological event, but was at least susceptible of profound 

 influence on zoological evolution, for it brought in new and richer supplies of 

 food in the form of seeds, fruits, and fodder. At present, neither the ferns, 

 equiseta, cycads, nor conifers furnish food for any large part of the animal 

 Ufe. The seeds of the conifers are indeed much eaten by certain birds and 

 rodents, but their foliage is httle sought by the leading herbivores. The 

 introduction, therefore, of the dicotyledons, the great bearers of fruits and 

 nuts, and of the monocotyledons, the greatest of grain and food producers, 

 was the groundwork for a profound evolution of herbivorous and frugivorus 

 land animals, and these in turn, for the development of the animals that prey 

 upon them. A zoological evolution, as extraordinary as the phytological one, 

 might naturally be anticipated, but it did not immediately follow, so far as 

 the record shows. The reptile hordes seem to have roamed through the new 

 forests as they had through the old, without radical modification. The 

 zoological transformation may have been delayed because animals suited to 

 the proper evolution had not then come in contact with the new vegetable 

 realm; but with the opening Tertiary the anticipated revolution appeared, and 

 swept forward with prodigious rapidity. " (174-175) 



From the above it is evident that there is some support for the view that the 

 Cretaceous marks the beginning of the last great era, rather than the close of 

 the preceding one. Further support of this suggestion is furnished by the 

 transition from the Cretaceous to the Tertiary, as sketched by Chamberlin and 

 SaUsbury (1906:3:161): 



"The Cretaceous period is commonly said to have been brought to a close 

 by a series of disturbances on a scale which had not been equalled since the 

 close of the Paleozoic era, and perhaps not since the close of the Algonkian. 

 These changes furnish the basis for the classification which makes the close of 

 the Cretaceous not the close of a period merely, but the close of an era as well. 

 While these changes are commonly said to have taken place at the close of the 

 Cretaceous, it is probably more accurate to say that they began late in the 

 Upper Cretaceous and continued into the succeeding period. The close of 

 the Cretaceous may be said to have been the time when these changes first 

 made themselves felt profoundly. They consisted of deformative changes, a 

 part of which were orogenic, and of igneous eruptions on an unprecedented 

 scale (161). 



"Transition beds between Mesozaic and Cenozaic. — ^In general, the Laramie 

 is conformable with the Montana below and unconformable with the Eocene 

 (Tertiary) above. The break between the Laramie and Eocene is locally a 



