THE HABITAT OF THE SAUROPOD DINOSAURS 463 
thickness is about goo feet, and it is possible, if not probable, that 
beds are included in this measurement which are older than the 
Morrison. The range in thickness is usually from 700 feet in the 
western sections to less than too feet in the Black Hills region. 
This thickness is exceedingly slight for a formation of such vast 
geographic extent as the Morrison. In general, the western sections 
are thicker than the eastern, but this will not hold as an invariable 
rule. Sections of 400 feet or less sometimes occur in the western 
areas, and sections fully 400 feet thick exist in eastern New Mexico. 
No section of more than 500 feet is known from the eastern areas, 
however, and the western sections frequently reach that or a 
greater thickness. The Morrison sediments might perhaps be 
described from the point of view of thickness as a thin mantle of 
sandstones and clays extending over a vast area, thickest in the 
west and thinning out definitely, but irregularly, to the east. 
Cross-bedding is abundant in the beds of the Morrison, espe- 
cially in the sandstones. It is represented by the type described 
by Walther and others as typical of desert deposits, and also by the 
type usually assigned to stream deposition. 
4. The stratigraphic relations of the formation are in a broad 
way disconformable with regard to.the underlying terranes. The 
deposits rest upon older formations of various ages, from the 
Unkpapa sandstone of uppermost Jurassic or earliest Comanchean 
age in the Black Hills region to Archean crystallines in the Rocky 
Mountain region. The relation of the Morrison to the overlying 
sediments appears to be a conformable one. The fact that the 
Morrison appears to be closely related to the succeeding formations 
has been pointed out by Lee.’ For further description of the 
Morrison formation the reader is referred to the above-mentioned 
article on the Morrison by the writer and to the bibliography 
contained therein. 
Taken together, the physical characters of the Morrison indicate 
a history something like the following: After an extensive period of 
erosion, during Jurassic and perhaps late Triassic time, Western 
North America was invaded from the north by the sea, and the 
tW. T. Lee, ‘‘Reasons for Regarding the Morrison an Introductory Cretaceous 
Formation,” Bull. Geol. Soc. Amer., XXVI (1915), 303-14. 
