WALCOTT.J TEXAS. 217 



A more extended account of the geology and the paleontology was 

 published by him in 1852, in which he notes the occurrence of Silurian 

 rocks in Llano County, on the San Saba River (p. 7) ; and describes 

 and illustrates Lingula acutangula (p. 90), Pterocephalia sanctl sabce (p. 

 92), and the genus Pterocephalia. Attention is called by notes and 

 figures to two undescribed species of trilobites. 1 



In a letter addressed to the Secretary of the St. Louis Academy of 

 Sciences in 1859 Dr. B. F. Shumard states that he had discovered in 

 Burnet County, Texas, an extensive development of Lower Silurian 

 rocks, equivalent to the Potsdam sandstone and the Calciferous sand- 

 rock of the New York system. The Potsdam sandstone is tilled with 

 trilobites belonging to the old genus Arionellus, with Obolus and Lin- 

 gula, and a small Or this, and it rests directly upon the grauite. Above 

 the Potsdam we have beds which appear to represent the third mag- 

 nesian limestome and second magnesian limestone of the Missouri sur- 

 vey. 2 Two years laterJDr. Shumard published a more detailed account 

 of the Primordial zone of Texas, accompanied by a number of sections 

 measured in Burnet County, aud a description of new species of fossils 

 from limestones referred to the Primordial. Under the heading of "Pots- 

 dam sandstone" three sections are given ; one, 5 miles northwest of 

 the town of Burnet, has a thickness of 275 feet. In this fossils were 

 abundant in the upper portion, just beneath the Cretaceous limestone, 

 as well as in the lower beds. The base was not seen. This section is 

 made up almost entirely of limestones, as are the other two sections 

 mentioned. 3 



Dr. Shumard states that the Potsdam sandstone is also finely dis- 

 played in the southern part of San Saba County and around the mar- 

 gins of the granite districts of Llano. " The fauna of this division of 

 the Primordial zone of Texas is very analogous to that of the Potsdam 

 sandstone of Iowa, Wisconsiu, and Minnesota." The genera identified 

 are: " Dikelocephalus, Bathyurus, Arionellus, Conocephalites, Agnos- 

 tus, Liugula, Discina, Orthis, Camerella, Obolus, and Capulus, all of 

 which genera have been discovered in the Primordial sandstones of the 

 northwest except Agnostus and Camerella." 4 In the latter part of the 

 paper six new species were described, and a species of Camarella and 

 one of Capulus mentioned. In a letter to M. Barrande, dated Septem- 

 ber, 1860, Dr. Shumard describes in a general way the Primordial rocks 

 of Texas as seen along the Colorado River in Burnet County. 5 



Attention was called in 1874 to the presence of a sandstone in Organ 

 Mountains, close to the city of El Paso, in southwestern Texas, by Mr. 



'Die Kreidehildungen von Texas und ihre organischen Einschlusse. Bonn, 1852, pp. 92, 93. 



8 Letter touching the late discovery of Lower Silurian rooks, equivalent to the Potsdam sandstone 

 and Calciferous sandrock of the New York system, in Burnet County, Texas. Trans. Acad. Sci. St. 

 Louis, vol. 1, 1859, pp. 672, 673. 



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

 1861, pp. 216, 217. 



«Op.cit.,pp.217,21?. 



•Letter on the Primordial of Texas. Soc. geol. France, Bull., 2" ser., vol. 18, 1861, pp. 218, 219. 



