412 THE CAMBRIAN. [bull. 81. 



It is evident that Prof. Bradley had in mind the Tennessee section, 

 with which he was familiar, and on finding a quartzite, correspond- 

 ing to the Chilhowee quartzite, and then a sandstone and shale corre- 

 sponding in position to those of the Knox section, he correlated the two 

 upon the evidence of serial relation of similar deposits and the presence 

 of certain fossils in the limestone superjacent to the sandstone. 



Fortieth parallel survey. — The correlations made by the fortieth paral- 

 lel survey of the Paleozoic rocks of Utah and Nevada with those of the 

 eastern part of the continent, especially the New York section, are 

 based almost entirely upon the paleontologic evidence as determined 

 by Messrs. Hall and Whitfield. Within the area of the survey corre- 

 lations were made both on the stratigraphic succession of beds of dif- 

 ferent lithologic character, and the presence of similar organic remains. 

 A correlation based on the u likeness of two great masses of rock to 

 each other that occur beneath a known horizon n is made by Mr. Clar- 

 ence King as follows : 



Comparing the quartzites and argillites with those of the Cambrian section in 

 Wales, the likeness is too great to pass unnoticed, and in view of the enormous de- 

 velopments of these low-lying rocks as compared with the Silurian lying above the 

 Primordial horizon, I have determined to draw a line at the upper limits of the Pri- 

 mordial period to include the uppermost members of the'Potsdam epoch, and to con- 

 sider the whole underlying conformable series as Cambrian down to the point of 

 their nonconformity with the Archean.' 



Explorations and surveys west of the one hundredth meridian. — The 

 correlations made in the publications of this survey on the Lower Paleo- 

 zoic rocks are maiuly by Mr. G. K. Gilbert in his report on the geology 

 of portions of Nevada, Utah, California, and Arizona. 2 The data used 

 for correlation are, the stratigraphic position, lithologic characters, and 

 the presence of fossils of previously determined range. No attempt is 

 made to correlate by a study of the minor details of the faunas and 

 strata except in the correlation of the Tonto of the Grand Canon dis- 

 trict with the Primordial formations of the East. From the known 

 position of the genus Cruziana he argues that the beds containing it 

 are Lower Silurian or Primordial. Mr. F. B. Meek compares the species 

 of Olenellus found in central Nevada with 0. thompsoni and 0. vermon- 

 tana of Vermont and thus indirectly correlates the faunas. 3 



Geological surveys of the Territories. — The correlations made by the 

 various writers of the Powell and Hayden surveys were principally 

 based upon the paleontologic determinations of Mr. F. B. Meek and 

 Dr. C. A. White outside the territory of the survey, and within its ter- 

 ritory on stratigraphic positions, lithologic character, and fossils when 

 present. In most instances the data for correlation are not given, the 

 identification of the formations following that of previous workers in 

 the same territory. An example of this is found in the " Geology of the 



1 Paleozoic subdivisions on the 40th Parallel. Am. Jour. Sci., 3d ser., vol. 11, 1876, p. 476. 

 »Expl. and Surv. West of 100th Merid., vol. 3, 1875, pp. 156-171. 

 8 Op. cit., pp. 182, 183. 



