3901.] HATCHER MARIXH AND XOX-MAUIXI': FORMATIONS. 357 



Dakota sandstones, or the two succeeding intervals marked by the 

 deposition of the Judith river beds and the Laramie. 



After the deposition of some 300 to 400 feet of the sandstones, 

 shales and lignites of the Eagle formation true marine conditions 

 were again restored in this region. That this return to marine 

 conditions was gradual and due to a slow advance of the sea over 

 the region in question is evidenced in certain places by the manner 

 in which the uppermost Eagle sandstones pass by a series of shales 

 and lignites into the overlying true marine beds of the Claggett 

 formation, and is well shown on Eagle Creek a short distance above 

 where it empties into the Missouri river, some twenty miles above 

 Judith, Mont., at a place which may be regarded as the type local- 

 ity for the Eagle formation. 



Although we have not been able to trace this formation continu- 

 ously to the east and south and to observe its actual passage in those 

 directions into the overlying and underlying marine formations, yet 

 we do know that it disappears in these directions and is entirely 

 replaced by marine deposits resembling those of the Niobrara and 

 the base of the Montana. From all the evidence at hand it seems 

 probable that over the region now occupied by the Eagle formation 

 shallow water and in part terrestrial conditions prevailed for a con- 

 siderable period, commencing toward the close of the Colorado and 

 continuing uninterruptedly well up into the Montana, and that this 

 formation should be correlated with the upper portion of the Colo- 

 rado and the lower portion of the Montana. 



After the deposition of the Eagle formation marine conditions 

 again prevailed over this region, as is evidenced by the 300 to 400 

 feet of characteristically marine deposits known as the Claggett 

 formation intercalated between the Eagle sandstones and the Judith 

 river beds. From the nature of the deposits constituting the 

 Claggett formation it is evident that this return to marine condi- 

 tions was not so complete as it had been in the Benton, for at inter- 

 vals in the Claggett there are beds of sandstones with marine inver- 

 tebrates, indicative of shallow water or littoral conditions. These 

 are especially prevalent toward the top, where they may be regarded 

 as prophetic of that return to terrestrial conditions which was soon 

 to follow and which witnessed the deposition of the overlying 500 

 to 600 feet of non-marine deposits known as the Judith river beds. 

 As remarked earlier in this paper the passage from the Claggett 

 formation to the Judith river beds is in most places extremely 



PROC. AMER. PHI LOS. 800. XLIII. 178. X. PRINTED DEC. 7, 1904. 



