THE CRETACEOUS PERIOD 749 



largely of the shells of foraminifera; but with them are associated 

 shells of other types, some of which are similar in the two forma- 

 tions, and some dissimilar. The echinoderms, the sponge spicules, 

 and the secretions of certain microscopic plants 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. 



The Montana series. Following the Colorado epoch, there were 

 changes in the sedimentation and in the life of the western interior 

 sea. The Montana series is chiefly clastic, but the area of sedi- 

 mentation was somewhat contracted. The beds are, for the most 

 part, marine, and the water shallowed as the epoch progressed. 

 Local beds of coal give evidence of marshy conditions. Like other 

 parts of the Cretaceous system of the west, the Montana series 

 abounds in concretions, 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. The Ripley formation of the Gulf region is of about the same 

 age, probably, as the Montana series. 



The Laramie. 1 Deposition continued in the Great Plains and 

 to some extent west of them through the last epoch of the Cretaceous 

 period, but most of the sedimentation was non-marine. Fresh-and 

 brackish-water beds are widely distributed. The Laramie series 

 may be said to record the transition from the marine conditions of 

 the Montana epoch, to the fresh-water and land conditions of the 



1 For a full discussion of the Laramie (up to 1892) see White (C. A.), Bull. 

 82, U. S. Geol. Surv. A brief statement by the same author is found in the 

 Proc. A. A. A. S., 1889, Vol. XXXVIII. See also folios of the Great Plains 

 'region, U. S. Geol. Surv. 



