282 S : 1'. EMMONS — OROGRAPHIC MOVEMENTS. 



absent, was probably to a great extent covered by the Laramie deposits, 

 which may also have covered a great part of the present area of the 101k 

 mountains and of the White river plateau. 



< >n this method of reasoning, therefore, it would appear that already in 

 Laramie time the ocean waters had in great measure receded from the in- 

 terior portion of the Rocky Mountain region which they had occupied in the 

 earlier part of the Cretaceous period, hut that this recession was accompanied 

 by do dynamic movements. These movements were initiated only after the 

 coal-bearing Laramie beds had been deposited, and whatever sediments were 

 formed in the region after these movements were laid down in lacustrine 

 watei s. 



1 1 Ue qf il<< Movement. — I have spoken of this movement as post-( Iretaceous, 

 although, as occurring at the last stage of that series, it might more strictly 

 hr .idled post-Laramie. Twenty years ago the former term might have been 

 objected to as fixing too early a date for the movement ; to-day there seem- 

 to he some danger of a similar objection heing made to it on the ground 

 that it implies too late a date. All geologists are more or less Familiar 

 with the controversy which existed so long as to the age of this important 

 formation, which carries almost all the economically valuable coal deposits 

 of the Rocky Mountain region. It arose mainly from the fact that in the 

 earlier explorations fossils were brought in from widely separated districts 

 whose stratigraphy under the circumstances could not he exhaustively Btudied ; 

 hence correlations had necessarily to be made on palaeontologies.] evidence 

 without that accurate knowledge of the stratigraphical succession and struct- 

 ural nlatioii- of the beds in question which is an indispensable basis for the 

 correct determination of horizons in a new geological field. The determi- 

 nations made by various classes of specialists under these conditions presented 

 a wide range for the same series of beds. By the vertebrate palaeontologists 

 the Laramie was considered without doubt of Cretaceous age. From a 

 study of its mollu8Can remain.- opinion.- varied between Cretaceous and 

 Tertiary, with a decided leaning toward the latter; while the palseobotanists 

 assigned some of its beds to the Miocene and others to the upper Eocene, 

 the former heing in actual stratigraphical position nearesl the base of the 

 -■■i ii 



The geologists of the Fortieth Parallel, who first introduced in the western 

 mountain region systematic examinations of continuous areas based on topo- 

 graphic maps of these area.-, after following Laramie outcrops in a belt one 

 hundred miles wide across eight degrei - of longitude, found that Btatigraph- 



ically and structurally il belongs to the Cretact s, forming the closing 



phase of a continuous sedimentation through thai period, and being followed 

 by the mosl marked physical break .-inc.- that at the close of the Archaean. 



