13 THE CAMBRIAN. (bull. 81. 



established is adopted and then arbitrarily assumed whenever the orig- 

 inal basis for it is cleared away by more perfect knowledge. 



For convenience of description and geologic cartography, the geologic 

 formation 1 is taken as the primary unit of classification. The com- 

 bination of these primary units forms the terrane, which may be com- 

 posed of any number of geologic units. The groupings of the terranes 

 form the groups, and the union of the groups the systems. Thus the 

 Potsdam sandstone of the Adirondacks, the Dutchess County limestone 

 of New York, the upper Knox shales of Tennessee, the Hamburg shales 

 of Nevada, and many other formations unite to form the Upper Cam- 

 brian terrane. The union of the Upper Cambrian with the Middle and 

 the Lower Cambrian builds up the Cambrian group. The combination 

 of the Cambrian, Silurian, Devonian, and Carboniferous, forms the 

 Paleozoic system. 



The nomenclature of the formations of any group frequently ex- 

 presses in local terminology the history of the development or evolu- 

 tion of knowledge of the group. For instance, the formations compos- 

 ing the Cambrian begin, in America, with the " Granular Quartz" and 

 then follow the Potsdam sandstone, the St. John shales, the Chilhowee 

 sandstone, etc. The plan of naming the formations from the locality 

 at which they are best developed is a great safeguard and can be fre- 

 quently adopted. For instance, the Potsdam sandstone of New York 

 has its type locality at Potsdam in St. Lawrence County, where it can 

 be readily identified. To say It iS"l?Tentical withTmTUpper Cambrian 

 or St. Croix sandstone of the Mississippi Yalley, with which it has been 

 identified, and to speak of the latter as the Potsdam sandstone, is to 

 practically state that they are synchronous. The use of the name St. 

 Croix does not imply this. It enables the geologist to mention the 

 formation without assuming a correlation that he may or may not be- 

 lieve in. 



J Dr. J. J. Bigsby states that "According to Deshayes ' a formation is a space of time represented by 

 a certain number of beds, laid down under the influence of the same phenomena, 1 or, in other words, a 

 formation is characterized by the constancy of its fossils." (On the Paleozoic basin of the State of 

 New York. Part II. Classification of the Paleozoic strata of the State of New York. Quaurt. Jour. 

 Geol. Soo. London, 1858, vol. 14, pp. 428, 430.) 



