MIDDLE AND UPPEE ORDOVICIAN. 163 



The exact stratigraphic relations of this formation are not at present understood. The 

 only area in which it has been carefully studied is in the southern part of Montgomery and 

 Polk counties, where it outcrops over an east-west belt about 3 mUes wide, and from which it 

 thins northward until it is only a few inches to a few feet thick where it outcrops on the north 

 slope of Caddo Mountain. That the formation is absent north of Caddo Moimtain is known, 

 for the horizon at which it occurs is widely exposed. Whether the sudden appearance and 

 rapid thickening as we pass southward is due to its having been put down in a trough, or to 

 an erosion interval during which that part to the north was removed, will require fiirther field 

 work to determine; but its much contorted condition and its relations to the overlying forma- 

 tion, later to be described, impress one with the strong conviction that probably it is due to 

 an erosion interval. * * * 



From recent work it appears that the age of the Missouri Mountain slate, the Arkansas 

 novaculite, and the Fork Mountain slate can not yet be determine'd, for no fossils have been 

 found in them"* by recent workers in the area. They may be of Ordovician age, they may be 

 Silurian, and they may be Carboniferous. Their lithologic character is so very different from 

 the nearest rocks of Devonian age (coarse-grained sandstones and black shales), which occur 

 in northern Arkansas, that the possibility of their being of that age is scarcely entertained. 

 On the other hand, their siliceous character might easily permit them to be considered con- 

 temporaneous with either the Ordovician limestones of the Ozark region, which contain a great 

 deal of chert, or with the Boone formation (Mississippian) of that region, which is largely and 

 in parts entirely chert. The representative of the Silurian in northern Arkansas is the St. Clair 

 limestone, which contains no chert. * * * 



The Missouri Mountain slate, where the Blaylock sandstone is present, rests upon that 

 formation; where that is absent, it rests upon the Polk Creek shale at all points where contacts 

 were observed. While it partakes of the large and comparatively gentle folds of the area, it 

 appears not to enter into the minute crumpling of the Blaylock sandstone and lower formations, 

 but on the contrary to rest upon their upturned and what seem to be eroded edges. Whether 

 it does or not has thus far eluded aU efforts to determine, on account of the great amount of 

 d6bris nearly always present on the slopes where the contact between the two formations would 

 otherwise be exposed. 



This formation varies from 50 feet and less to 300 feet in thickness, the thinnest part being, 

 so far as observed, along the southern border of the Ouachita Range. The formation is a 

 clay slate. A few feet of the basal portion usually is green. Locally this lower portion is 

 somewhat sandy and such parts contain small crystals of iron sulphide. The remainder, above 

 the basal portion, varies from a blood-red to a dark red, containing patches and streaks^ of 

 green. The green at the base may be the original color, but that above is secondary. * * * 



The Arkansas novaculite is the principal formation of the Ouachita Range. Owing to an 

 unconformity at the top, its thickness is quite variable, but the maximum observed is about 

 800 feet. The character of the formation varies from the base to the top, as well as in different 

 parts of the same horizon. In a general way it may be described as consisting of a lower, 

 heavy-bedded siliceous part that is wholly novaculite and an upper, thin-bedded part of the 

 same material, interpolated with thin layers of black soft shale. In places there is an intervening 

 portion containing sandstone. 



The lower portion is about 300 feet thick and consists of massive beds from 2 to 10 feet 

 thick, with large ripple marks showing along the bedding planes. This part of the formation 

 furnishes the abrasives that are known in the markets as the Ouachita and Arkansas stone. 

 It is a very close textured rock, the grains being of microscopic size. The common color is a 

 very light gray, but bluish tints and pink, black, and dove-colored spots are quite common 

 in the lower 50 to 75 feet. The fracture is uneven to conchoidal, and it is very brittle. In 



a Prof. L, S. Griswold, in the report of the Geological Survey of Arkansas, vol. 3, 1890, pp. 404-407, names several 

 localities in which graptolites were supposedly obtained from above the novaculites. Several of these localities were 

 visited by Mr. H. D. Miser, who reports that in each case the graptolites occur above the Bigfork chert instead of above 

 the novaculite. 



