walcott.] CORRELATION. 411 



culiar products above, supposing there had been no overturning of the strata at the 

 particular localities. 



Hence, although paleontology does not enable us to ascertain the exact actual ages 

 of rocks, wheu applied with due caution and skill in connection with a careful ob- 

 servance of their stratigraphical arrangement and lithological and other physical 

 characters, it does afford the means of fixing their relative ages, as well as of identify- 

 ing the same beds at different localities, within given fields of observation, with very 

 considerable precision. It is therefore not merely one of the more important aids to 

 the geologist in his investigations, but in the present state of geological science it 

 is the only sure guide in classifying and determining the order of succession of rocks 

 where this can not be done by their actual continuity or obvious superposition. For 

 these reasons it is now the universal practice in all geological surveys conducted 

 upon sound scientific principles to devote especial attention to this department. 



ROCKY MOUNTAINS. 



Whitney. — In speaking of the occurrence of the Primordial fauna in 

 Nevada Prof. J. D. Whitney calls attention to the fact that the fossils 

 are of Potsdam age, and that similar trilobites and brachiopods occur 

 in Bohemia iu argillaceous shales; throughout the United States, from 

 New York to the Rocky Mountains, in the " Potsdam sandstone, or in 

 the shales or slates ; v in Texas and in Nevada, in limestone. He says : 



This discovery will also indicate the necessity of caution in theorizing on the geo- 

 logical structure and age of regious which have only been hastily examined. It will 

 not do to put down every red-sandstone group below the Trias in the far west as 

 "Potsdam," for that subdivision may be much lower down, hiding itself as a modest 

 limestone of a very neutral tint of color. 1 



Although not stated, it is evident that the principle of correlation 

 used is purely biologic, or the occurrence of similar fossil remains. 



Bradley. — When describing the strata upon the western base of the 

 Wasatch range, near Ogden, Utah, Prof. F. H. Bradley refers the 

 quartzite, some 1,500 feet in thickness, to the Potsdam, and the gray and 

 calcareous shales and limestones above to the Upper and Lower Silu- 

 rian. 2 A reference to the Upper Silurian was on the assumption that 

 the limestone was the same limestone as that found 25 miles to the 

 north, in which the characteristic Niagara coral Halysites catenulata was 

 obtained by Dr. Hayden in 1871. The data upou which the other cor- 

 relations are made are not given. It is probable and almost certain 

 that the presence of a quartzite beneath the limestone, supposed to be 

 of Silurian age, was considered to be sufficient for the reference to the 

 Potsdam sandstone. He correlates the glauconite sandstone beneath 

 the limestone of the Teton range section, which carries Conocoryphe 

 and Dikelocepbalus, with the Knox sandstone of Safford, in Tennessee. 

 Beneath them, and often present when they are absent, he found from 

 50 to 75 feet of a very compact ferruginous quartzite " which must rep- 

 resent the Potsdam." 3 



1 Note on the occurrence of the '■ Primordial Fauna" in Nevada. Am. Jour. Sci., 3d ser., vol. 3, 1872, 

 p. 85. 



2 Report of Frank H. Bradley, geologist of the Snake River Division. U. S. Geol. Sur. of the Terr., 6th 

 Ann. Rep. 1873, p. 194. 



»0p.cit.,p.216. 



