CRITERIA FOR DETERMINING THE ORIGIN 319 



that deposition took place in a number of small lakes, with deltas on the 

 borders, and possibly by rivers as well. Hatcher maintained that the 

 deposits were laid down in a series of tloodplains, with alternating deposi- 

 tion and erosion at successive levels. 



In discussing this subject several considerations are of prime impor- 

 tance, other factors being accessory. The important facts to be considered 

 are these : 1, the formation was laid down on a comparatively level sur- 

 face; 2, the fauna is exclusively of the continental type, either land or 

 fresh water ; 3, the nature of the sediments is such as to imply deposition 

 in or by quiet water at times, and again in more agitated waters; 4, the 

 thickness of the formation is much greater in the west, and especially 

 the southwest, than in any of the other Morrison areas, gradually thin- 

 ning out toward the east and northeast; 5, internal structures, such as 

 channeling; 6, the "uniformly variable" character of the succession of 

 beds. Other factors to be considered are : the presence and kind of cross- 

 bedding, the condition of preservation of the fauna and flora, the varie- 

 gated colors, etc. 



Conclusions 



From these facts as starting-points we may infer that the Morrison 

 was deposited on a wide-spread plain of low relief, and probably of low 

 altitude. It is the result of alternating deposition and erosion, there 

 being no place, probably, where deposition went on continuously from 

 the time when the first beds were laid down until the uppermost beds 

 were deposited. The source of the material was to the west, and espe- 

 cially the southwest, of the present area of its outcrops. 



These conditions may liave been satisfied by such, a history as the fol- 

 lowing: III .liirassic time there was a crustal disturbance and slight up- 

 heaval ill the present Rocky Mountain area. This disturbance is shown 

 ill tlie iiiuhilating character of the lied Beds noticed by Lee in northern 

 New iMexico. Following this upheaval, erosion progressed steadily until 

 much of the liocky Mountain area was reduced to a fairly level plain. 

 At the end of the interval, when the mountains to the west were fairly 

 well reduced in height and extent, the Rocky Mountain plain, if it may 

 be so called, was low and flat and the site of numerous lakes and swamps. 

 Erosion in the plain itself was no longer ])ossible to any considerable 

 exlcnl. Erosion al' the inoiiniaiiKnis areas to the west was sfill [jossilile, 

 however, mid went foruanl siradily. It is the ]ii(Mhicts of tiiis later 

 erosion wliicb imw coniprisc the Moiiisdii fdinial ion. A lew large 

 streams llnwing caslward or northeastward Iroiii (lie nld mountains. 



