322 C. C. MOOK ORIGIN AND DISTRIBUTION OP THE MORRISON 



Such a history as outlined above will fit the conditions observed in 

 the Morrison formation, and it is probable that its history was something 

 of that nature. 



It is important to observe that a given thichness of beds deposited 

 under the conditions of alternate deposition indicated would reciuire a 

 much longer time for its formation than beds deposited under conditions 

 of continuous deposition. It is perfectly in accordance with the above 

 outlined history for parts of the formation to be Upper Jurassic in age 

 and for other parts to be distinctly later than the Jurassic, perhaps well 

 up in the Comanchian. It could even be that practically ail the beds 

 represented in a single outcrop of the formation might be Jurassic, and 

 that in another area, not a great distance otf, be made up largely of beds 

 of Comanchian age. The accompanying diagram is an attempt to show, 

 in a veiy schematic way, the kind of cross-section the fonuation would 

 have immediately after deposition and before being covered or disturbed. 



If the above interpretation be correct, great care must be taken in 

 judging the age of the formation as a whole, from a fauna or flora of 

 definite age, in any one locality or level. It will only be possible to de- 

 cide definitely the age of the various parts of the formation when ex- 

 tensive collections have been made from a number of levels in many 

 localities. 



