THE EOCENE PERIOD. 205 



principal sources of sediment and the sites of its lodgment shifted 

 somewhat from time to time, and among the scattered deposits referred 

 to this period, there are notable differences of age. Several more or 

 less distinct stages of deposition have been recognized, the distinctions 

 being based partly on the superposition of the beds, and partly on 

 the fossils which they contain. 1 These several stages are not readily 

 correlated with those of the coastal regions, since synchrony is not 

 readily established between formations containing marine fossils on 

 the one hand, and those containing terrestrial fossils on the other. 



1. The oldest recognized stage of the Eocene in the western interior 

 is the Fort Union (perhaps corresponding to the Midwayan, p. 199). 

 During this stage, there was an extensive area of aggradation in parts 

 of North Dakota 2 and Montana, and a still larger area in Canada, 

 where the sediments which constitute the Fort Union beds were 

 deposited. These beds, composed of sand, clay, etc., are said to be 

 locally 2000 feet or more thick, and have usually been described as 

 lacustrine. The presence of fresh-water shells (unios, etc.), is consist- 

 ent with this conclusion for some parts of the formation; but the 

 abundance of the leaves at many places is quite as suggestive of sub- 

 aerial aggradation for other parts. 3 



The Fort Union beds overlie the Livingston formation (p. 159) 

 conformably, 4 and have been thought, on the basis of their fossils, 

 to represent the oldest Eocene formations of the interior. It will 

 be remembered however that the youngest formations referred to 

 under the Laramie (Arapahoe, Denver, Livingston, etc., p. 15S), were 

 deposited in fresh water or brackish lakes, or on land, and that their 

 reference to the Laramie instead of the Eocene, is of doubtful pro- 

 priety. At any rate, the time of terrestrial aggradation, so character- 

 istic of the Cenozoic era in the western part of North America, had 



1 For an account of the deposits near the 40th parallel, see King's Report, Vol. I, 

 already cited. For the latest attempt at correlating the several lake formations, 

 see Dall, 18th Ann. Rept., U. S. Geol. Surv., Pfc. II. See also J. H. Smith, Jour. 

 Geol., Vol. VIII, pp. 444-471. 



2 Wilder has recently called into question the separability of the Fort Union and the 

 Laramie, in western Xorth Dakota. Jour, of Geol., Vol. XII, p. 290. 



3 For criteria for distinguishing lacustrine and subaerial formations, see Davis, 

 Science, X. S., Vol. VI, p. 619, 1897, and Proc. Am. Acad. Arts and Sci., Vol. XXXV, 

 p. 345, 1900. 



4 Little Belt Mountain, Mont., Folio, U. S. Geol. Surv. 



