162 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



The maximum observed thickness of the limestone lenses is about 75 feet. They are 

 made up of layers from a few inches to 2 feet in thickness and they may thin down to a few 

 feet or entirely disappear, within very short distances. The rapid thickening and thinning 

 of the limestone, its sporadic occurrence, the conglomerate it contains, and the lithologic differ- 

 ence between the shale of the formation, and that beneath led to the conclusion in the field 

 that there is an unconformity between the two formations, which afterward was confirmed on 

 paleontologic evidence by Mr. E. O. Ulrich. The irregular occurrence of the limestone is due 

 to its having been put down in the valleys of the old land surface, and the pebbles of the con- 

 glomerate at the base of the limestone doubtless were carried in by the streams flowing over 

 that surface. 



The upper or shale portion of the formation usually is from 50 to 75 feet thick. It is a 

 very black shale and is soft enough to soil the fingers in handling, qualities due to the presence 

 of a large per cent of finely disseminated graphite. In places the graphite is conspicuous and 

 occurs in thin, wavy sheets. While most of this part of the formation is shale, there sometimes 

 occur in it, especially near the top, thin beds of dark-colored chert, very similar to the Bigfork 

 chert, next to be described, as well as thin layers of limestone. Some of the shale layers are 

 quite calcareous, and when the lime is dissolved out from these, there remains a gray to pink 

 colored porous, spongy shale, fragments of which are frequent on the slopes. Fossil graptolites 

 abound in the shale of this formation and occttr sparingly in the limestone. 



The Bigfork chert is a very close textured, even-bedded siliceous rock, in layers from 1 to 

 18 inches thick, but the most common thickness is from 3 to 6 inches. * * * The color 

 varies from a slate to a dark gray, the former being most common. It is very brittle and 

 under the blows of the hammer flies into small pieces. The fracture is angular, in some of 

 the layers approaching conchoidal. In places it is thickly set with a network of fine quartz 

 veins. It is along these veins that the stone breaks when struck with a hammer, and so numer- 

 ous are they that it often is difficult to secure a hand specimen with fresh surfaces. Straight 

 joints several inches in length with remarkably smooth surfaces are common. Weathered 

 portions have the appearance of fine-grained gray weathered sandstone. Usually the layers 

 are crumpled to an astonishing degree, and it probably was the strain accompanying this 

 intense folding that caused the network of thin joints, which subsequently were filled with 

 quartz, forming the veins above described. Its thickness is about 700 feet. 



This formation is mapped by the Geological Survey of Arkansas " as novaculite, but it 

 differs materially from the true novaculite of the area in being of a coarser texture, darker 

 color, thinner bedded, not translucent, older, and in having a much more complex structxtte. 



The Polk Creek shale overlies the Bigfork chert, with which it is conformable, and outcrops 

 along the bases of many of the ridges in the Ouachita Range. In color, hardness, and texture 

 this shale is much like the Ouachita shale. It resembles that also in containing a large number 

 of quartz veins and locally in having slaty cleavage well developed. In this, as in the Ouachita 

 shale, graptolites abound. It differs from that shale in containing no sandy or calcareous 

 layers and in being only about 100 feet thick. In places it appears to be absent. 



The Blaylock sandstone is well exposed at the eastern end of the mountain, where the 

 Little Missouri River cuts across the exposed edges. Like the other formations described, it 

 is extremely crumpled. The repetition of beds resulting from the crumpling renders its thick- 

 ness difficult to determine, but this is thought to be about 1,500 feet in the thickest part. ' Appar- 

 ently it is conformable with the Polk Creek shale beneath. In parts, through a hundred feet 

 or more, it consists almost wholly of sandstone, while in other parts it is made up of alternating 

 beds of sandstone and shale. The sandstone is in layers that usually are from 1 to 6 inches 

 thick, and the bedding is very even. Some of the layers are quite quartzitic and contain 

 numerous thin quartz veins. Others of the thin layers closely resemble chert. It is fine 

 grained to medium grained, and varies from dove-colored to dark gray or green. GraptoHtes 

 occur in it rather sparingly. The interbedded shale is_ dark colored, often black, and fissile. 

 In places it contains large numbers of graptolites. 



oGeol. Survey Arkansas, vol. 3, 1890. 



