FORMATIONS AND PHYSICAL HISTORY 537 



interior by the sea, the invasion going so far, probably, as to estab- 

 lish connection between the Gulf of Mexico and the Arctic Ocean, 

 over tin- site of the Great Plains. Clastic formations predominate 

 in the Colorado series as a whole, but there are beds of chalk com- 

 parable to those of Europe, from Texas to South Dakota. The 

 abrogate thickness of the series is locally as much as 3,000 feet, as 

 strata are measured, though its average thickness is much less. 



The origin of chalk. There has been much difference of opinion 

 concerning the origin of chalk. Its resemblance to the foraminif- 

 eral ooze of the deep seas long since led to the belief that it was a 

 deep-sea deposit; but closer examination has thrown doubt on this 

 conclusion, for the differences between the chalk and foraminiferal 

 ooze are as striking as their likenesses. Both consist largely of 

 the shells of foraminifera, but with them are associated shells of 

 other types. The echinoderms, the sponge spicules, and the secre- 

 tions of certain microscopic plants of the chalk correspond in a 

 general way with those of the oozes now forming, and are consistent 

 with the deep-water origin of the chalk. The molluscan shells of 

 the chalk, on the other hand, seem to point with clearness to water 

 no more than 30 to 50 fathoms deep. The distribution of the chalk 

 and its relations to other sedimentary beds indicate its deposition 

 in shallow water, not in water comparable in depth to that in which 

 oozes are now formed. On the whole, the balance of evidence is in 

 favor of the view that the Cretaceous chalk was deposited in rela- 

 tively shallow water. The conditions for its origin seem to have 

 been clear seas, with a genial climate. Its materials may accumu- 

 late as well on the bottom of a shallow sea as on the bottom of a 

 deep one, if clastic sediments are absent. 1 



Following the Colorado epoch there were changes in the sedi- 

 mentation and in the life of the western interior sea. The Mon- 

 tana series is chiefly clastic, but the area of sedimentation was 

 somewhat contracted. The beds are largely marine, and the water 

 shallowed as the epoch progressed. Land formations also are found 

 in the series. Local beds of coal give evidence of marshy conditions. 

 Like other parts of the system, the Montana series abounds in con- 

 cretions, some of which attain great size. The thickness of the series 

 is variable, and its maximum is great. From 8,700 feet in Colorado, 

 it thins to 200 feet in some parts of the Black Hills. 



Deposition continued in the Great Plains and to some extent 

 1 For fuller statement of this subject see Earth History, Vol. Ill, p. 149. 



