218 THE CAMBRIAN. [bull. 81. 



W. P. Jenney. He states tbat the quartzite rests upon a coarse felds- 

 pathie granite, and passes upward into sandstone, tilled with Scolithus 

 linearis, which he identified as the Potsdam sandstone. The superjacent 

 formation is a crystalline limestone identified as calciferous. It con- 

 tains several species of fossils resembling Archseocyathus, and rarely a 

 gasteropod, like Pleurotomaria. 1 In commenting upon the statements 

 of Mr. Jenney, Prof. J. S. Newberry says: 



Now it appears there are found in the Rio Grande Valley the crystalline base. 

 rocks overlaid in succession by the Potsdam and Primordial, the Trenton, Hudson, 

 Upper Siluriau(f), and Carboniferous, just as they occur in the Northern and Eastern 

 States. * * * It would seem as though the Organ Mountains had formed one 

 border, and the New York and Canadian highlands the other, of the great Paleozoic 

 invasions of the sea. 3 



A section of the strata referred in 1874 to the Potsdam by Mr. S. B. 

 Buckley, at Packsaddle Mountain, in Llano County, gave a thickness of 

 385 feet of alternating limestone and sandstone, beneath which occurs 

 a massive bed of reddish brown sandstone 326 feet in thickness. Numer- 

 ous genera of Primordial fossils were found, characteristic of this hori- 

 zon. 3 Other references are made to the occurrence of the Potsdam 

 sandstone at various localities in this county. In the report for the 

 following year Mr. Buckley refers to the occurrence of sandstones of 

 the Potsdam period in Mason and Menard Counties. He also calls 

 attention to the probable presence of rocks of Silurian age at the base 

 of the Organ Mountains at Fort Bliss, near the town of El Paso. 4 



A preliminary report of a reconnaissance of a portion of the Paleo- 

 zoic area of central Texas was made by Mr. C. D. Walcott in 1884. A 

 measured section of the strata referred to the Cambrian on the west 

 side of Honey Creek Valley gives 245 feet of sandstone and 625 feet of 

 limestone, all marked by the presence of an abundant Upper Cambrian 

 (Potsdam) fauna. The upper beds of the limestone become compact, 

 hard, and have a little included cherty matter. The Cambrian fauna 

 terminates here, as far as observed, and it is not until over 1,000 feet of 

 limestone are passed through that recognized fossils again occur. The 

 fauna is then of the type of the Calciferous group. 5 The text is accom- 

 panied by a diagrammatic section of Packsaddle Mountain, that illus- 

 trates the unconformity between the Llano group and the strata re- 

 ferred to the Potsdam or Upper Cambrian. This gave a total thick- 

 ness of 605 feet for the formation, with a massive sandstone 205 feet in 

 thickness at the base. 6 



The latest addition to our knowledge of the Cambrian rocks of Texas 



1 [Recent explorations in the Geology of Texas.] New York Lyceum Nat. Hist. Proc, vol. 2, 1874, 

 p. 69. 



2 Remarks on geology of Western Texas. New York Lyceum Nat. Hist.. Proc. vol. 2, 1874, p. 70. 

 •First annual report of the geological and agricultural survey of Texas. Houston, 1874, p. 73. 



4 Second annual report of the geological and agricultural survey of Texas. Houston, 1876, p. 10. 

 6 Notes on Paleozoic rocks of Central Texas. Am. Jour. Sci., 3d ser., vol. 28, 1884, p. 433. 

 6 0p.cit.,p.432. 



