MOOK, 8TUBY OF THE iMORRISON FORMATION IQl 



HisTOKY OF Previous Opinions as to the Origin of the Formation 



The origin of the Morrison formation has been the subject of consider- 

 able attention in the past. Some of the more important discussions are 

 noted at this point. Their merits will be discussed later. 



C. A. White (1886, 2) in discussing the Morrison and its invertebrate 

 fauna came to the following conclusions : "The character of the strata in 

 which these fresh-water Jurassic fossils were found, both at the Colorado 

 and at the Wyoming localities, in addition to the character of the fossils 

 themselves, is such as to indicate for them a lacustrine, and not an 

 estuary or a iluviatile, origin; that is, the rocks are regularly stratified 

 and have such an aspect and character as to indicate that they were 

 deposited in one or more large bodies of water. If the strata of the 

 Colorado and of the Wyoming localities really contain an identical fauna, 

 it may be regarded as probable that they were deposited in one and the 

 same lake. The distance between the Colorado and the Wyoming locali- 

 ties indicates that the supposed lake was nearly 200 miles across; and, 

 if the Black Hills fossils also belonged to the same contemporaneous 

 fauna, the assumed lake was much larger. ^^ 



Eiggs (1901, 4) described the Morrison of the Grand Eiver Valley, 

 and gave an interpretation of the history of deposition as follows : "i^et 

 us attempt to trace the history of the Jurassic formation as evidenced by 

 the nature of the rocks, the stratigraphy and the occurrence of fossils: 

 Given an arm of the Jurassic sea, fed by rivers and open to the ebb and 

 flow of tide waters. Under these conditions the sediments washed down 

 by the river ever^'where accumulated slowly, and alternating with them 

 thin ledges of limestone and gypsum were laid down. Occasional strata 

 of sand accumulated by the action of the retarded currents about the 

 estuaries of streams. Later, by some change in levels, the ingress of 

 seawater was cut off, but the outlet still remained and so ensued the 

 gradual change from salt to fresh water. Then followed a period of 

 comparatively uninterrupted deposition in which the green shale was laid 

 down under still water. Along with it were deposited near the mouths 

 of streams the occasional homogeneous beds of green sand. As the basin 

 filled up and its outlet deepened, the lake became shallower until its bed 

 was invaded by the shifting channels of broad and shallow streams. Its 

 sand-bars have formed the cross-bedded sandstone ledges which mark the 

 transition from the lower to the upper clays. With the shallower waters 

 came the great land and shore reptiles and about the estuaries of streams 

 their remains were deposited abundantly. 



"Again the lake waters invaded the region and the deposition of sand 



