12 PREFACE. 



geologic introduction in which the Colorado formation is defined, many- 

 local sections are described, and faunal lists are given that show the 

 vertical range and areal distribution of the most of the species. Inci- 

 dentally, some new evidence concerning the position of the Bear River 

 formation is given, and large areas of Cretaceous strata in Utah are 

 correlated with the formations east of the Rocky mountains more accu- 

 rately than has heretofore been possible. 



In addition to the aid received from Dr. C. A. White and the free 

 use of the collections under his curatorship in the U. S. National 

 Museum, I am under many obligations to Dr. W. H. Dall and to Mr. 

 Charles T. Simpson, of the National Museum, for assistance in compar- 

 ing fossils with recent species, and for valued opinions concerning the 

 generic relations of doubtful forms ; also to Prof. Alpheus Hyatt, who 

 has examined several of the species of Ammonitince at ray request, and 

 has given me the benefit of his authoritative opinion as to their rela- 

 tionships. 



T. W. S. 



