CAMBRIAN AND LOWER ORDOVICIAN. 83 



to 6,000 feet, and includes the whole of the upper Cambrian and the Calciferous of the Ordo- 

 vician. From the base of the formation upward to the top of the middle Cambrian the rocks 

 are composed of thin-bedded and, in part, intraformational conglomerate and shaly limestones. 

 While pursuing stratigraphic field work in connection with the study and collection of the 

 fossils, Mr. E. O. Ulrich noted the occurrence of what appeared to be shaly lentils at the top' 

 of the middle Cambrian, indicating to his mind an unconformity between the upper and lower 

 Cambrian. 



Beginning with the upper Cambrian the rocks are composed of massive and hard, pink to 

 yellow crystalline limestone and dolomite, which weather to various shades of brown and almost 

 black. Rocks of this character have a thickness of 500 to 600 feet, and are followed by lighter- 

 colored massive dolomite and limestone, without any indication of time interval, up into the 

 Ordovician. Beginning approximately 450 feet below the top of the Arbuckle formation the 

 limestones become more thinly bedded and are associated with thin and, in places, shaly strata. 

 Very near the top occasional sandy beds occur. 



In the massive beds in the central part of the formation fossils are found, but not in great 

 abundance. Near the top, however, where the rocks become argillaceous, fossils are more 

 common. Fossils of a number of genera, mostly undescribed, occur in the upper 1,250 feet 

 of the Arbuckle limestone. 



The Reagan sandstone and the Arbuckle limestone in the Wichita Mountains 

 of Oklahoma are described by Taff ^°^ as follows : 



The Reagan sandstone is the lowest Cambrian formation in the Wichita region, and it 

 rests on the eroded uneven surface of the granite porphyry, from which most of its materials 

 have been derived. * * * 



The Reagan sandstone is approximately 300 feet thick and is composed of hard and soft 

 sandstone, grit, conglomerate, shales, and siliceous shell limestones. The section of the forma- 

 tion is essentially the same as in the western end of the Arbuckle Mountains. In each case 

 porphyry is beneath it, and the limestone which overlies the Reagan is of the same nature and 

 contains the same fossil fauna. In both locahties the Reagan formation is made up of con- 

 glomerate composed of porphyry pebbles and included basic rocks, gritty light-brown to gray 

 and greenish sandstones, greenish clay shales, and siliceous limestones, inters tratifie'd. The 

 limy layers contain many species of Cambrian fossils, which were carefully collected, but which 

 have not yet been thoroughly studied. The conglomerate occurs invariably near the base as 

 local beds or lentils, while the calcareous sandstone and limestone beds are without exception 

 in the upper part of the forination. * * * 



Above the Reagan formation in conformable succession is a great limestone section essen- 

 tially the same as the Arbuckle formation in the Arbuckle Mountains, where the lower part,, 

 practically the lower third, was found to be Cambrian and the upper two-thirds Ordovician 

 in age. Near the transition zone, between the rock of the two periods, very few fossils have: 

 been found, so that precise distinctions as to age can not be made at the present time. Thei 

 same conditions of deposition seem to have occurred in the Wichita region as in the Arbuckles, 

 and the rocks must be classified in the same manner. There seems to be no reason to doubt 

 that the formation is continuous from one region to the other beneath "Red Beds" deposits. 



E. 0. Ulrich contributes the following note regarding the Reagan sandstone: 



The Reagan sandstone includes most of the Cambrian deposits exposed in the uplifts of 

 the Arbuckle and Wichita mountains, in south-central and southwestern Oklahoma, respec- 

 tively. The formation as described by J. A. Tail" and as observed by me consists of two mem- 

 bers. The lower is a brown sandstone, coarse grained and iU bedded below; finer grained, , 

 laminar, and shghtly calcareous above; varj-ing greatly in thickness because it rests directly 

 on the uneven pre-Cambrian granite floor. The upper member, though not sharply separated 



a Tishomingo folio (No.. 98), Geol. Atlas U. S., U. S. Geol. Survey, 1903. 



