GENERAL OBSERVATIONS. 83 



The Point of Rocks Group is well represented in the collections by 

 molluscan species, all of which are either Conchifers or Gasteropoda. Among 

 them are a few land shells, many fresh- water species, and all but a few of 

 the remainder belong to genera of known brackish-Avater habitat. Further- 

 more, some species of these remaining few genera have been found asso- 

 ciated with brackish-water forms. It thus appears that the formerly pre- 

 vailing- marine conditions over the region that we "now call the Plateau 



O O 



Province began to draw to a close early in the epoch represented by this 

 group, and had well nigh ceased at its close. 



A good illustration of this gradual change is presented by the strata of 

 the Point of Rocks Group at Upper Kanab, Utah, where, toward the base 

 of the series exposed, the fossils are mostly marine; above these a greater 

 proportion of brackish-water forms are introduced; still higher up fresh-water 

 forms prevail, and the upper strata contain only fresh water and land shells; 

 the deposition of all the strata having evidently been continuous and uninter- 

 rupted. 



Among the more interesting observations that have been made in rela- 

 tion to the fresh water and land shells of this epoch of the Cretaceous period, 

 are those concerning the great differentiation of types that had thus early 

 taken place. To so great a degree had this differentiation then attained 

 that the species of Unio, Helix, Pliysa, &c., seem to have been as diversified 

 and well developed as they are at the present time. Indeed the species of 

 these genera are so closely like some of those now living that they need 

 only the fresh condition of recent shells to remove all suspicion of their 

 great antiquity from the mind of the casual observer. 



After the foregoing statements concerning the faunal characteristics of 

 the Point of Rocks Group, one might naturally inquire for the reasons that 

 have led to its reference to the Cretaceous rather than to the Tertiary period. 

 Laying aside all considerations suggested by the vertebrate and floral remains 

 that have been collected from its strata, the reply may be briely stated thus: 

 There is no physical break between this group and the Salt Wells Group 

 below it. Its strata contain at least three species of Inoceramus, which 

 genus has never been known in strata of later date than the Cretaceous 

 period. Odontolmsis, a species of which has been obtained from nenr the 

 summit of the group, is regarded as a Cretaceous genus; and in view of the 



