158 ANNALS NEW YORK ACADEMY OF SCIENCES 



thickness is usually about 200 feet, but in some cases it is as high as 300 

 or as low as 150 feet; in the Black Hills area the thickness is never more 

 than 200 feet, and ranges from that down to zero, averaging about 100 

 feet; in the southeastern Colorado and New Mexico areas the thickness 

 varies from 400 to 200 feet. It will be seen from this that while there 

 is a considerable variation in individual areas, the thickness is much 

 greater in the western areas than w the eastern, and that there is a 

 thinning out eastward, ivhich is very gradual considering the distances 

 involved. 



4). The size of grain of the sediments varies to a considerable extent. 

 Pine-grained material is the most abundant; a considerable amoimt of 

 medium-grained material is present, with a smaller amount of coarse- 

 grained rock. Very coarse sediment does not appear to be present. 

 Coarse material is more abundant in the western than in the eastern 

 areas. 



5). The succession of beds varies greatly from point to point, but the 

 kind of succession is j)ractically the same in every locality. Fine joint- 

 clays or grits usually make up the greater part of the outcrops, especially 

 toward the top. Sandstone and nodule layers are usually present at 

 intervals in the section, and often thin limestones or occasionally con- 

 glomerate beds. These various members are found to be arranged in a 

 certain order in one, locality, and in another locality not far away the 

 succession will be different. Sandstone beds that are thick in the first 

 section may be thin or absent in the second. Limestone beds present in 

 a certain position in the first section may be in another position in the 

 second section, or absent altogether. On studying the various sections 

 of the formation a constant thinning and thickening or replacement of 

 individual members is to be seen. An examination of the sections in 

 the stratigraphic division of this paper will emphasize the prevalence of 

 the conditions above described. This type of succession in many sections 

 has been described by Lee as "uniformly variable." 



6). The contacts of the various members with each other is usually 

 sharp, showing rather rapid changes of conditions of deposition. Beds 

 of fine grits or clays are often followed directly by beds of medium- 

 grained sandstone and vice versa. 



7). Channeling is often present, certain layers lying in troughs eroded 

 in the underlying beds. This has been discussed in the section on 

 structures. 



8 ) . Cross-bedding is common in the Morrison. Both the stream and 

 wind types have been observed. 



