PHYSICAL CONSIDERATIONS 309 



description here, and it is perliaps sufficient to state thai its character 

 and distribntion have been consideied sufficient to indicate that it was 

 probably deposited over floodplains on a nearly flat surface, in lagoons 

 and temporary lakes, and in marshes along sluggish streams. The phys- 

 ical peculiarities of the Morrison a it so striking and the fossils char- 

 acteristic of it have been found in so many places that there is little 

 difficulty in identifying it. 



STRUCTURAL RELATIONS 



] wish In call pai'ticular attciilioii to the stiiict iiral I'clations of the 

 ^Morrison with the formations below and al)o\e it. At some of the locali- 

 ties vt^here the Morrison has been examined there is an abrupt lithologic 

 change from it to the rocks on ^\■hich it rests. This suggests a thne 

 break, although in most places there is no obvious discordance in dip; 

 Init the reality of the hiatus becomes apparent when we find the Morrison 

 overlapping older formations that range in age from Jurassic to Archean. 



In most places the bedding planes of the Morrison aie so nearly parallel 

 with those of the formations above and l)elow it that the structural rela- 

 tions can only be appi'ehended by taking a broad view. In some ]>laces 

 in A\'yoming, northwestern Colorado, and I; tali the Morrison rests on 

 marine Jurassic, and in other places on older rocks. In western Colorado 

 the Gunnison is conspicuously unconformable on the older formations 

 (8, 9), the upper part, or direct equivalent of the Morrison, overlapping 

 the lower part, or possible efiuivaleiit of the La Plata sandstone. In 

 eastern Coloiado it rests on roeks of Triassic or Carboniferous age gen- 

 erally, but in some places, as near Pikes Peak (12) and east of the (rreen- 

 horn ^fountains (13), it overlaps onto the pre-Cambrian granite, lu 

 northern New Mexico it overlaps the I'^xeter and rests unconformably on 

 the Tiiassic. hi brief, the iclations of the Morrison to the underlying 

 rocks are \aried. and the niieniiroi mity below it denotes much more time 

 in some places than in otheis. 



In strong coiitfast wilh (be uiiconrormable relations at its l)ase, the 

 Morrison is obviously conforinable w ith the lieds above it. In relatively 

 few places has the I'urgatoii'e, or ibe lower part of the so-called Dakota, 

 been proved to overlap the ^Fon^ison, and so many of the reported over-- 

 laps have be(>n found erroneous (!•) that there seems to be no good rea.'^on 

 for postulating any considerable time break in the liocky ^lountain region 

 at the top of the Morrison, alilunigh some geologists have regai'ded such 

 a break as necessary in comparing the IJocky Mountains with other i-egions 

 (5, page 109). 



