4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



the fossiliferous limestone, and other beds which had formed in 

 the sea and near it. The correlation of the fossiliferous marine 

 beds with the gypsiferous strata is regarded as the chief contri- 

 bution offered in this paper, for by this means a narrow zone 

 of rocks, whose age is determined by means of fossils of marine 

 organisms, may be followed far beyond the limits of the fossiliferous 

 beds into unfossiliferous sedimentaries whose age has been in doubt. 

 The area in which these beds are exposed is large and difficult to 

 traverse. It may be many years before this tracing is done and until 

 it is done physiographic data seem to furnish the best available 

 means of correlating the unfossiliferous beds from place to place. 

 The events following the withdrawal of the Jurassic sea are better 

 known than the preceding events. In the epoch next following this 

 withdrawal, the final stages of planation of the Rocky Mountain 

 region were accomplished and on the extensive plain the sluggish 

 streams formed bayous, swamps, and temporary lakes, and spread 

 out the sediments of the Morrison formation, building up a plain 

 which seems to have been almost perfectly graded from New Mexico 

 to Montana, and from central Utah to eastern Kansas. Over this 

 graded plain advanced the waters of the Lower Cretaceous sea and 

 later those of the great submergence in Upper Cretaceous time, 

 which covered the site of the Rocky Mountains and buried their 

 roots beneath its sediments, where they remained dormant until 

 stirred to life by the post-Cretaceous or post-Laramie movement, 

 when the present Rocky Mountains began to rise. 



PRE-MESOZOIC PHYSIOGRAPHY 



PENNSYLVANIAN SUBMERGENCE 



It is not my purpose to consider the geographic conditions of the 

 Rocky Mountain region, prior to the Mesozoic era, further than is 

 necessary for a proper understanding of Mesozoic physiography. 

 During much of the Carboniferous period sea water covered large 

 portions of the area now occupied by the mountains. Marine lime- 

 stone of Pennsylvanian age is abundant in central and northern 

 New Mexico and in central and western Colorado. It has been the 

 belief of many geologists that open sea conditions prevailed in western 

 America during the time that the coal measures were forming in the 

 eastern and central parts of the continent. Statements are frequently 

 made that " in the western part of the United States there are no 

 coal accumulations of this age (Pennsylvanian)." * There is unques- 



1 Schuchert, Qias., Textbook, p. 745. 



