MIDDLE AND UPPER OEDOVICIAN. 159 



Near the eastern boundary of Arizona, in the Clifton district,^^^'^ the Longfellow 

 limestone represents the early Ordovician ("Calciferous," Ulrich) and possibly 

 some Upper Cambrian, but the later Ordovician is lacking. 



The Ordovician of New Mexico is described by Gordon and Graton ^'^ as 

 comprising the equivalent of the lower Ordovician and also limestones which carry 

 a distinct Richmond (uppermost Ordovician) fauna. Fuller descriptions are given 

 by Lindgren.'*^ 



I 14. OKLAHOMA. 



In the Arbuckle Mountains of Oklahoma Ordovician strata are represented 

 by the Arbuckle limestone (Upper Cambrian and early Ordovician; see pp. 82-83, 

 Chapter III), the Simpson forhiation (of Chazy or Stones River age), and the Viola 

 limestone (of Black River to Richmond age). Taff and Ulrich ^°^"- give the follow- 

 ing account of the stratigraphy and correlation : 



After the Arbuckle limestone was deposited there was a general change in the nature of 

 the sediments. The top of the hmestone seems to have been slightly eroded locally and upon 

 the surface were deposited beds of pure sand. At other places the Arbuckle limestone is over- 

 lain by shaly and impure lirne — the basal beds of the Simpson formation. On these local sandy 

 beds at the base there were deposited greenish shales and thin crystalline and shelly limestones 

 interstratified with a number of beds of sandstone, making a total thickness ranging from about 

 1,200 to 2,000 feet. 



A generalized section measured on the south side of the Arbuckle uplift gives 

 the following beds : 



Feet. 



Thin limestone and shale 890 



Sandstone 100-200 



Thin-bedded limestone and shale, with some sandstone 1,250 



The sandstone separates the formation into two divisions, regarding which 

 Taff and Ulrich say : 



The fauna of the lower division of the Simpson formation is decidedly similar to that of the 

 Chazy of New York and Canada and of the Pogonip formation of Nevada. Considering these 

 east and west connections, it is surprising to note that the lower Simpson fauna, so far as known, 

 contains none of the prolific fauna of the lower division (Murfreesboro limestone) of the Stones 

 River group in middle Tennessee, which is beUeved to be of equivalent age. As a whole, the 

 fauna of the upper part of the Simpson is closely related to the upper division of the Stones River 

 group in Tennessee and Kentucky and the equivalent beds in the upper part of the Mississippi 

 VaUey. 



The description of the Viola limestone follows and comprises the items quoted 

 below : 



There is a gradual transition from the top of the Simpson formation up into the basal part 

 of the Viola formation. The top layers of the Simpson are usually argillaceous, are thinner, 

 darker, and more evenly stratified than those at the base of the Viola, and in places contaim 

 partings and beds of clay. The rocks at the contact on each side, however, vary somewhat in 

 different parts of the field. 



The Viola formation is a continuous but slightly variable deposit of limestone 500 to 700 

 feet thick, which usually appears massive on very fresh exposures. Upon weathering, however, 

 the bedding is pronounced, showing layers rarely more than a foot in thickness. In the lower 

 and middle parts especially there are occasional bands and nodular masses of chert. 



The Viola limestone is divisible into three members, distinguished by variations in bedding, 

 in texture, and to a less extent in the color of the limestone. The upper and middle members 



