GEOLOGIC HORIZONS AND LIFE ZONES. 



17 



and antelope-like forms, point to a plains or open valley environment in this region in middle Miocene time. The 

 deposits in which the faunas occur are of types which suggest that they were laid down under subaerial conditions as waste 

 slope and playa lake deposits; the evidence for this view is corroborated by the occurrence of the mammalian remains, 

 which are usually scattered and frequently much gnawed by rodents." 1 (p. 84.) Phillips Ranch is regarded by Matthew 

 (1910) as near Sheep Creek age, namely, Lower Middle Miocene. 



Phillips Ranch, Lower Levels of Monolith Series, Tehachapi, Southern California. 



This California horizon is regarded by Buwalda, Merriam, and Matthew as early Middle Miocene, Merychippus 

 zone. It may be slightly older than the Sheep Creek of Nebraska as no types have been found here so hypsodont as the 

 more progressive Sheep Creek mutations. The isolated molar teeth of Merychippus described by Buwalda are relatively 

 short-crowned or intermediate in length. 



Phillips Ranch, Age of. — " The horse material from Phillips Ranch resembles still less any of the horses found in the 

 Ricardo Fauna, being much more primitive. It is likewise less advanced than the horse species obtained in the Tejon 

 Hills in the southern San Joaquin Valley. The Phillips Ranch fauna certainly represents a stage of evolutionary develop- 

 ment much earlier than the Barstow, Ricardo and Tejon Hills [Chanac] faunas. " 1 (p. 82.) 



Fig. 9a. Geologic distribution in ascending order of mutations of Merychippus isonesus 



from Sheep Creek. 



Amer. Mus. 14184, type of M. isonesus quartus Osborn; Amer. Mus. 141S5, type of M. isonesus quintus Osborn; 

 Am. Mus. 14181, specimen referred to M. isonesus quartus Osborn; Am. Mus. 14180, type of M. isonesus tertius Osborn; 

 Am. Mus. 14179, type of M . iso?iesus secundus Osborn; Am. Mus. 14187, type and topotypes of M. isonesus primus Osborn, 

 the most primitive form. 



Virgin Valley, Nevada. 



Of the same age as Mascall or slightly more recent. Merriam (Trans. Am. Philos. Soc, 1915, p. 210) observes that 

 the Mascall and Virgin Valley formations of the Great Basin region are approximately of the same age as or older than 

 the "Temblor." 



This horizon described by Merriam 2 contains referred specimens of Parahippus, Merychippus isonesus, Hypo- 

 hippus fosborni, H. fequinus; also remains of Moropus ref. and Merycodus furcatus ref. 



1 Buwalda, John P. "New Mammalian Faunas from Miocene Sediments near Tehachapi Pass in the Southern Sierra Nevada." 

 Univ. Cal. Publ. Bull. Dept. Geol., Vol. 10, No. 6, Nov. 18, 1916, pp. 75-85. 



2 Merriam, John C. op. cit. 



