STRATIGRAPHIC SUMMARY. 609 



2,500 ft. Below it is the Hiwassee slate 700-1,500 ft. thick and 

 below this the Snowbird formation 350-5,000 ft. thick, and resting 

 unconformably upon the Precambric crystallines. These basal 

 Cambric beds are probably all of continental origin. 



In central Texas, the Cambric section begins generally with the 

 Middle Cambric, though Lower Cambric has been thought to be 

 present. Comstock* divides the Cambric of Texas into : 



III., Katemcy series. 



II., Riley series. 



I., Hickery series. 



In the Rio Grande region of New Mexico and Texas, the Bliss 

 sandstone (300 ft.) and the Shandon quartzite are referred to the 

 Cambric. In Oklahoma the basal Cambric is the Regan sandstone 

 of Middle Cambric age, which here rests by overlap on the Pre- 

 cambric Tishomingo gneiss. It is from 50^500 ft. thick, and passes 

 upward into the Arhuckle limestone, which ranges in age from 

 upper Middle Cambric at the base, to Lower Ordovicic at the top, 

 Further north, in the Ozark region, the basal Cambric begins with 

 the La Motte sandstone which rests unconformably upon the Pre- 

 Cambric. It and the succeeding Bonneterre limestone represent 

 the Middle Cambric, while the succeeding Elvins formation and 

 Gasconade limestone represent the Upper Cambric. The latter, 

 together with the overlying Rouhidoux and Jefferson City limestone 

 of Lower Ordovicic age, has been grouped as the Potosi or Yell- 

 ville limestone series. In the upper Mississippi valley, the Upper 

 Cambric alone is represented in the Saint Croix formation, which 

 is locally subdivided as follows (Berkey) : 



SUPERFORMATION. 



Oneota dolomite (Ordovicic). 

 Saint Croix formation (Upper Cambric). 

 5. Jordan sandstone. 

 4. Saint Lawrence dolomite and shales. 

 3. Franconia sandstone. 

 2. Dresbach shale. 

 I. Hinckley sandstone. 

 {Unconformity.) 



* First Annual Report, Geol. Survey Texas, 1889-90. 

 39 



