354 THE CAMBRIAN. [bull. 81. 



Potsdam really belong within the Upper Cambrian horizon. From our 

 present knowledge the Upper Cambrian zone is the only portion of the 

 group represented in Colorado. 



The Cambrian rocks included within the States of Colorado, Wyo- 

 ming, Montana, and South Dakota are of the type of those of the south- 

 ern Interior Continental, Upper Cambrian series, and consist usually of 

 a bed of quartzite or conglomerate, resting unconformably upon the 

 Algonkian or Archean, and passing aoove into calcareous beds that 

 frequently carry the Upper Cambrian or Potsdam fauna. This is the 

 case with the formation about the Big Horn Mountains, Wind River 

 Mountains, and the Teton and Gros Ventre ranges of Wyoming. In 

 some instances, however, as about the Black Hills, a fauna occurs in the 

 basal sandstone similar in character to that of the St. Croix sandstone 

 of the upper Mississippi Valley. Traces of a fauna were fouud in Colo- 

 rado, at Quandary Peak, in a schistose rock that is supposed to occur 

 just above the quartzite. 



SOUTHWESTERN SUB-PROVINCE. 



The widely separated areas of Cambrian rocks in Central Texas and 

 northwestern Arizona are united in one sub-province on account of their 

 similarity in stratigraphic position, general sedimentation, and faunas. 



TEXAS. 



With the exception of the doubtful occurrence of the Upper Cambrian 

 zone as a sandstone in the Organ Mountains of southwestern Texas, as 

 mentioned by Mr. W. P. Jenney, the Cambrian formation is, as far as 

 known, confined to the Paleozoic uplift in the central part of the State 

 that includes Llano County within its central portion. 



A section of the formation measured by Dr. B. F. Shumard in Bur- 

 net Couuty, 5 miles northwest of the town of Burnet, is as follows : l 



Feet 



No. 1. — Soft, chalky limestone with well marked cretaceous fossils 50 



No. 2.— Gray and bluish gray hard limestone, of a sandy texture, in beds from a 

 few inches to a foot thick, and containing Arionellua (lUiihyurus?) 

 planus, Baihyurus depressus, Camerella sp., Orthis color adoensis, Orthin 



sp. ?, Distinct microscopica, and Lingula sp. ? 81 



No. 3. — Mottled gray, purple, and greenish, earthy and subcrystalline limestone, 

 with alternating bands of siliceous limestone — contains the samefossils 



as No. 2 45 



No. 4. — Slope with projecting ledges of No. 3 55 



No. 5. —Slope 24 



No. 6. — Schistose coarse textured limestone, made up of crystalline particles and 



abounding with fragments of trilobites, chiefly Arionellus 20 



Total 275 



1 The Primordial zone of Texas, with descriptions of new fossils. Am. Jour. Sci., 2d ser., vol. 32 1861, 

 p. 216, 



