During the Cretaceous Period 221 



wasps, butterflies, and moths that aid in cross pollination 

 of flowers which bear colored floral parts. 



The extensive and intercontinental volcanic disturbances 

 already referred to as occurring toward the close of the 

 Lower Cretaceous, not only started marked invasions of the 

 sea in some places, they seem also to have so created bar- 

 riers against the ocean, and to have so diverted long river- 

 courses, that great inland lakes were created, in which an 

 abundant fish-life might flourish. One of the most extensive 

 and earliest of these in the Cretaceous was the centre for 

 deposition of the freshwater strata of the Dakota forma- 

 tion in western North America. Regarding this Chamber- 

 lin-Salisbury (5:111:144) say: "The Dakota formation 

 is present over the great plains generally, though buried 

 over the greater part of the area. It extends westward 

 beyond the eastern ranges of the western mountains, though 

 in the mountain region, the area of deposition was greatly 

 interrupted by elevations which rose above the lakes, 

 marshes, or river flats where the sedimentation took place. 

 In Northern Montana, it is not known west of the Rocky 

 Mountains. The original eastern boundary of the forma- 

 tion is not known, for erosion has removed it from con- 

 siderable areas which it once occupied. Remnants of the 

 formation are now exposed, as far east as eastern Iowa 

 and Minnesota. It must have originally covered an area 

 1,000 miles wide and 2,000 miles long, within the United 

 States." 



"The Dakota formation has commonly been regarded 

 as a lacustrine formation, deposited during an epoch of 

 crustal oscillation, during which the depth of the basin in- 

 creased. The necessity for postulating numerous oscilla- 

 tions and nice adjustments, is largely removed, if the forma- 

 tion be regarded as the joint product of subaerial and 

 fluviatile deposition, for deposits of this class furnish their 

 own adjustments. The presence of bird-tracks in the 

 Dakota of Kansas, and the preservation of some 500 species 

 of plant fossils, mostly the leaves of angiosperms, at 

 various points and in conditions which forbid much trans- 

 portation, imply the prevalence of subaerial conditions to 

 a notable extent at least. 



