MIDDLE AND UPPEE ORDOVICIAN. 161 



is shown by the fossils they contain; and the Crystal Mountain sandstone, the formation imme- 

 diately above, is considered Ordovician on lithologic grounds. But careful search has failed 

 to reveal any fossils in the Collier shale, nor has it any lithologic characters that would give it 

 claim to an age classification with the overlying rocks. On the other hand, it is quite different 

 from the rocks above, and at its top there is at least locally a conglomerate which future study 

 may determine to be widespread. For these reasons, it is thought best not to even provisionally 

 place this with the Ordovician. It may be Cambrian. If so, these are the oldest outcropping 

 rocks in the State. * * * 



It is a dark, soft, graphitic clay shale, containing widely separated thia beds of dense black 

 and intensely fractured chert. As a result of the severe squeezing and shearing it has under- 

 gone practically all traces of beddiug have disappeared. It is intensely crumpled and is full of 

 glossy, slickensided sxu:faces. In places slaty cleavage may be observed. The upper 100 feet 

 or more is quite calcareous, limestone occurring in dark-colored crystalline lenses and layers 

 one-half inch or more in thickness, distributed through the shale. In other parts the lime- 

 stone is several feet in thickness and occurs in much contorted layers from a few inches to more 

 than 2 feet thick, interbedded with thin seams of graphitic shale. In such cases the lime- 

 stone is dark and compact, is much jointed, and on long exposure weathers in very uneven 

 surfaces. * * * 



The rocks of known Ordovician age within the Ouachita Range include five formations, 

 which are here known as the Crystal Mountain sandstone, the Ouachita shale, the Stringtown 

 shale, the Bigfork chert, the Polk Creek shale, and the Blaylock sandstone. 



The Crystal Mountain sandstone consists of two parts — a lower massive one composed 

 wholly of sandstone, 300 feet thick, and an upper part consisting of rather massive layers of 

 sandstone interbedded with black to gray shale, 400 feet thick. The shale is in places altered 

 to ribboned slate, like the lower part of the Ouachita shale. The lower part is the main rock 

 of the Crystal Mountains, which owe their existence to this massive, slow-weathering formation. 

 It is a coarse-grained white sandstone, composed of well-rounded grains, and commonly weathers 

 to the color of brown sugar. In many parts it is thickly set with a network of quartz veins 

 from the thickness of a knife blade to several inches. In other parts there are fissures from 

 several inches to several feet in width, the walls of which are lined with magnificent clusters of 

 quartz crystals. * * * 



This formation has not to the present time produced any fossUs, and it is considered of 

 Ordovician age wholly on lithologic grounds. The sandstone passes gradually into the Oua- 

 chita shale above, a,nd the close resemblance of the included shale beds to the Ouachita shale 

 seems to the observer in the field conclusive evidence that the two are of the same age. 



The Ouachita shale is the next formation above the Crystal Mountain sandstone, into 

 which it passes by gradual increase of sandy layers. * * * The formation is intensely 

 crumpled, and for that reason its thickness has not defioitely been made out, but it probably 

 is not less than 900 feet. 



For the most part this is a dark-colored clay shale, but not uncommonly the dark layers 

 alternate with green ones. In many places slaty cleavage is developed, when the alternating 

 layers of dark and green produce ribboned slate. Somewhere in the lower portion there are 

 thin layers of limestone, interbedded with the shale. Quartz veins and thin layers of hard 

 flinty material are common, and fragments of quartz frequently occm- on the ground where 

 this is the surface rock. The shale is frequently dissected by straight, weU-defined joints. 

 Graptolites, while not so common as in the shales above, are frequently to be found. 



The Stringtown shale in Arkansas rests unconformably upon the Ouachita shale and is 

 from 75 to 150 feet thick. It consists of two parts, a lower calcareous part and an upper shaly 

 part. The lower part of the formation contains lenses of bluish compact brittle limestone, 

 usually thickly set with calcite veins. The basal portion of this is sometimes conglomeratic, 

 the pebbles ranging from the size of peas to 2 inches in diameter and subangular. They consist 

 of shale, very fthe grained brown sandstone, and limestone. * * * 

 48011°— 12 11 



