GEOLOGIC HORIZONS AND LIFE ZONES. 



9 



to conclude with the progressive species of Merychippus, such as M. calamarius of the lower portion of the Santa Fe 

 marls and of Barstow, California. 



The Pliocene is here made to commence with -the appearance of the Procamclus-Hipparion fauna, so that it includes 

 the Ricardo of the Pacific coast, the Niobrara River near Ft. Niobrara, the Little White River of South Dakota, and 

 many other localities where the full Procamclus-Hipparion fauna has been found. Doubtless many of these formations 

 overlap the close of the Miocene and beginning of the Pliocene. 



This change has been made with the full concurrence of Matthew and Merriam and conforms with the previous con- 

 clusions of Scott. It does not as yet rest on the secure foundations of comparison with the marine Lower Pliocene fauna. 

 Yet Merriam observes (letter, February 28, 1917): "With reference to this point it may be stated that the evidence of 

 percentages of extinct species in the invertebrate Pliocene fauna of California seems very different from that of the latest 

 Miocene of Europe, and much more like the percentage in the Pliocene." These most recent views of Merriam are pre- 

 sented in the accompanying correlation sheets (Figs. 13, 16). 



The dates of the names assigned to formations, beds, etc., are taken chiefly from "North American Geologic 

 Formation Names" by F. B. Weeks. 1 The formations are grouped under life zones. 



NEBRASKA AND S. I IAKC >TA 



MONTANA 



Fig. 5. General correlation of the fossil bearing horizons of the western plains district and of Oregon. The "Loup 

 River" is now regarded as Lower Pleistocene, Equus excelsus-Elephas imperator zone. The Procamelus-Hipparion zone 

 is'now included in the Pliocene. Modified after Osborn and Matthew, 1909. 



"IlTANOlHIFltJM-KlEECElPPCS ZCNF. LtJDIAN (SUPER.), SANNOISIAN, STAMPIAN, OP EUROPE. 



1. Horizon A of Hayden and Leidy, 1869, lower part of the White River series, South Dakota. 



2. Titanotherium beds, South Dakota, Hayden and Leidy, 1869. 



3. Chadron formation, South Dakota, 200 ft., Darton, 1905. 



4. Horsetail Creek beds, northeastern Colorado and western Nebraska, Matthew, 1901. 



5. Monument Creek formation (upper part), Darton, 1905, 1906. 



6. Pipestone Creek beds, Thompson Creek, western Montana, Douglass 1899, 1901, 1903. 



7. Swift Current Creek, Cypress Hills, British Columbia, Cope, 1885. 



1 Bull. U. S. Geol. Surv. No. 191, 1902. 



