THE 



AMERICAN JOURNAL OF SCIENCE 



[FIFTH SERIES.] 



Akt. IV. — Gastropod Trails in Pennsylvanian Sand- 

 stones in Texas; by Sidney Powees. 



During a brief examination of the Pennsylvanian 

 section near Shafter, western Texas, in 1919, the tracks 

 here described were observed on the eastern side of Cibolo 

 Creek. Dr. J. A. Udden reported on the geology of the 

 region in 1904 and described the tracks as those of trilo- 

 bites ( f ) - 1 In the same report he described conglomerates 

 which he believed to be of • glacial origin. The drawings 

 of the tracks (figs. 1-3) have been made with great care 

 by Mr. G. S. Barkentin, of Albany, N. Y. The following 

 analysis of their origin has been made possible through 

 the careful study of Professor P. E. Raymond who is con- 

 tributing an accompanying paper on trails. The speci- 

 mens were given to the Museum of Comparative Geology. 



Shafter, a silver mining camp situated in the canyon of 

 Cibolo Creek, 19 miles north of Presidio on the Mexican 

 border, is shown on the Shafter topographic sheet of the 

 U. S. Geological Survey. It is in the midst of the Trans- 

 Pecos volcanic plateau and the exposures of Cretaceous 

 and Pennsylvanian sedimentary rocks are due to block- 

 faulting followed by deep dissection. The silver is largely 

 chloride and occurs as fissure and contact deposits in the 

 Cibolo limestone of Pennsylvanian age. The country 

 rock runs 1 to 3 ounces to the ton, average ore 10 oz., good 

 ore 80 to 90 oz. 



The geologic column is as follows : 



Pleistocene Bolson deposits. 



Tertiary Volcanics. 



Upper Cretaceous Volcanics, granitic intrusions ( ?) 



Lower Cretaceous Limestones. 



1 The geology of the Shafter silver mines district, Presidio County, Texas, 

 University of Texas Mineral Survey, Bull. 8, p. 60, 1904. 



Am. Jour. Sci. — Fifth Series, Vol. Ill, No. 14.— February, 1922. 

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