44 GEOLOGICAL BIOLOGY. 



beyond, took place chronologically at the same general 

 period, and that this series of disturbances may have affected 

 the whole of the northern hemisphere is further suggested by 

 the occurrence of gigantic erratic blocks of granite in the 

 midst of Eocene strata in the neighborhood of Vienna and 

 other places : Vezien * has suggested that an ice age is indi- 

 cated by them. 



The Division-line between the Cretaceous and the Tertiary. — 

 This Rocky Mountain revolution marks the period of the 

 second great break in the life of the geological ages. The 

 Mesozoic time began with the close of the Appalachian rev- 

 olution, and closed with the elevation of the marine Creta- 

 ceous beds . above ocean-level. In our classification the 

 division-line between the Cretaceous and the Tertiary was 

 arbitrarily placed at the top of the chalk formations conspicu- 

 ously developed on both sides of the British Channel. The 

 difficulty, which American geologists have found in drawing 

 the precise line to separate the Mesozoic from the Cenozoic, 

 has resulted from the change in the character of life of the 

 beds in the western interior from marine to brackish, fresh- 

 water, and land types. This change was incident to the 

 Rocky Mountain revolution, which had already begun and 

 was slowly lifting the whole region while the fresh-water 

 sediments were being laid down. Several stages may be 

 marked in this grand revolution, but the facts connected with 

 them are not so well developed as to serve for general purposes 

 of classification of the time-scale. The amount of elevation 

 produced by these epeirogenic movements after the deposit 

 of the marine Cretaceous, in the western half of our continent, 

 is estimated to have been not less than 32,000 or 35,000 feet.f 



Columbia River Lava Outflow. — At the close of the Miocene 

 a great outflow of lava in the northwestern part of the United 

 States took place, and continued with interruptions through 

 the Tertiary into the Quaternary time. About the Columbia 

 River, where it cuts through the Cascade range, the basalt is 

 over three thousand feet thick, and the outflows cover a vast 

 extent of territory, estimated at 150,000 square miles. This 



* Rev. Sci.. vol. xi. p. 171, 1877. 



f G. M. Dawson, Am. Jour. Sci., vol, XLix. p. 463. 



