268 S. I. EMMONS — OROGRAPHIC MOVEMENTS. 



peculiar lithological composition of the overlying Dakota Cretaceous which 

 renders that formation one of the most readily recognizable of all the Meso- 

 zoic series. As actual observation has shown that in some cases the earlier 

 loeists included these beds in their Dakota formation, the term Jura- 

 Dakota bas been used in this paper to designate the beds first deposited after 

 the movement, in order to distinguish them from the marine Jurassic beds of 

 other regions, which were deposited before them ; without, however, implying 

 thereby, in localities that have not been personally observed, more than the 

 probability of tin' existence of the freshwater beds. 



The evidence of this movement thus far obtained is of two kinds : First, 

 thai derived from personal observation in regions of violent disturbance, 

 where, during the elevation produced by the movement, considerable areas 

 had been uplifted by folding, often combined with faulting. and greal thick- 

 nesses of rocks, sometimes thousands of feet, bad heen eroded away ; and where, 

 during the subsequenl depression, dura-Dakota beds had been deposited 

 upon these eroded surfaces. The most marked evidences of such movements 

 are found in the Elk mountain region, where, along a single line of strike, 

 the dura-Dakota beds upturned during the post-Cretaceous movement 

 are seen to rest alternately and in repeated successions upon beds of all the 

 horizon.- from Archaean up to Trias, and to rest upon the latter in the middle 

 of the region in perfect conformity. Other violently disturbed region- ob- 

 served are the northern Mosquito range, the eastern flanks of the mount- 

 ains mar ( "anon ( 'ity. and tin' northern portion of the San Juan mountain-. 



'flu- second class of evidence is the tint indicated by geological maps that 

 the Dakota Cretaceous, presumably Jura-Dakota, rests directly upon Ar- 

 chaean or Carboniferous at very many points throughout the region. In the 

 other portions uf the region, where the dura-Dakota is represented as resting 

 ■ in the Trias, unconformity by erosion has in a few cases been detected. 



The most persistent and readily recognizable horizon of Mesozoic age in 

 the Rocky Mountains is the Dakota Cretaceous. It is prevailingly a sand- 

 stone with a characteristic basal conglomerate, the sand-tone becoming 

 readily quartzitic, even when adjoining Bauds tones are not altered, so t hat its 

 upturned strata, owing to their resisting nature, always stand out promi- 

 nently, 'fin- fresh- water" dura below it, bo far as it ha- been studied, gener- 

 ally ha- a sandstone at or near its ba.-e which is softer and frequently cr< 

 bedded to a remarkable degree. Between these t\\" sandstones i- a series of 

 -hah- ami clays, cairs ing a certain amount of limestone, which in some plai 

 forme a continuous bed, and at others occurs in lenticular bodies in the shall 

 The shales are frequently variegate. 1 in color, and bedsof gypsum are some- 

 times found. 



The Cretaceous beds above the Dakota consist, in the Fori lien tun group, 

 largely of .lark .-hah-.-, with a slighl development of limestone, often bitumi- 



