160 INDEX TO THE STEATIGRAPHY OF NORTH AMERICA. 



are approximately 300 feet thick. The lowest member is somewhat thinner and more variable 

 than the others. The lowest member consists of light-colored, coarse-textured, and usually 

 roughly bedded limestone. It probably wiU not exceed 100 feet in thickness anywhere, and in 

 places is much less. 



The middle member consists of white to Ught-blue and generally even- and thin-bedded 

 limestone. On account of the abundance of fossil trilobites of the genus Trinucleus in certain 

 layers it might be known as the Trinucleus member. It is characterized by white surface where 

 weathered and even bedding. * * * 



An investigation of the fossils by Mr. Ulrich, who collected and studied the rock section, 

 shows that there are three faunal divisions, which correspond to the three Hthologic members 

 above described. The most fossiUferous development of the basal member was noted at the 

 extreme northern occurrence, in the region at the base of Double Mound, 6 miles northeast of 

 Roff. A preliminary study of the fossils collected shows [a large number of species, nearly all 

 of which] occur in Phylloporina and Chtambonites beds of the Ordovician section in Minnesota. 

 These beds are correlated with latest Black River and earliest Trenton. Many of the species 

 occupy these horizons in New York and Canada and what are regarded as equivalent positions 

 in middle Tennessee and Kentucky. 



As a whole the middle member or Trinucleus bed of the Viola hmestone is not abundantly 

 fossiliferous. The organic remains occur chiefly in three horizons, one near the base, another 

 above the middle, and the third near the top. The first and second contain an abundance of 

 graptohtes. Next to the Trinucleus the graptohtes are the niost characteristic and commonest 

 fossils of the middle member. The fossils indicate that the Trinucleus beds were deposited 

 during the latter half of the Trenton age. 



The upper member of the Viola limestone is approximately 300 feet in thickness, but only 

 in the upper 25 feet is it abundantly fossihferous. The lower 50 feet yielded no fossils. In the 

 middle portion the fossils are not only rare but poorly preserved. The fossils are all species 

 characterizing the upper divisions of the Richmond age in Minnesota, Wisconsin, Illinois, 

 Indiana, and Ohio. In northern Arkansas the same fauna occurs in the Polk Bayou limestone 

 and in middle Tennessee in the Fernvale formation. 



I 15. ARKANSAS. 



Purdue '^^^ has recently published the results of studies of the stratigraphy of 

 the Ouachita Mountains, in Arkansas, as worked out by himself and Ulrich : 



Feet. 



Carboniferous: Stanley shale 6,000 



Unconformity. 



Age unknown : 



Fork Mountain slate 100 



Arkansas novaculite 800 



Missouri Mountain slate 300 



Probable unconformity. 



Ordovician: 



Blaylock sandstone ' 1,500 



Polk Creek shale 100 



Bigfork chert 700 



Stringtown shale. 100 



Unconformity. 



Ouachita shale 900 



Crystal Mountain sandstone ' 700 



Probable unconformity. 



Age unknown; Collier shale (observed thickness) '. 200 



11,400 



The Collier shale was considered by the Geological Survey of Arkansas as of Lower Silurian 

 (Ordovician) age.'' Some of the older rocks above this shale are certainly of Ordovician age, as 



''Geol. Survey Arkansas, vol. 3, 1890. 



