694 CHARLES R. KEYES 



have been made with ordinary care it will probably be rare that 

 radical alteration will be demanded. 



In the present stage of geologic inquiry it is neither prac- 

 ticable nor desirable to map all districts with uniform refinement. 

 Among geological formations, we shall no doubt eventually 

 establish and indicate on all geological maps about five degrees 

 of taxonomic rank. The division lines in any one area can only 

 be fixed after very careful comparisons with those of all the 

 neighboring districts. In the main, these terranal lines will be 

 found to correspond to, or can easily be adjusted to, the divisional 

 lines separating the lithologic individuals ordinarily recognized. 



When the lithologic features of formations gradually merge 

 into those of others, or when there is a rapid alternation of dif- 

 ferent kinds of rock layers, fossils or minerals, other criteria 

 may have to be resorted to in order to properly delimit the ter- 

 ranes. But this fact certainly in no way invalidates the general 

 principles involved in the recognition of the lithologic individual 

 as the leading object to be represented in cartography. 



When, for example, it was found upon detailed faunal exami- 

 nation 1 that the great St. Clair limestone of Arkansas, which 

 had long been considered by the workers of that state as a 

 single lithologic unit, was in reality two great limestones of 

 almost identical lithologic appearance, the one Ordovician in 

 age, and the other Silurian, it did not render worthless the maps 

 upon which these two terranes had been represented as a single 

 cartographic unit. Nor is the principle of mapping the litho- 

 logic individual to be given up on this account. Early observa- 

 tion was merely insufficient.- 



In the case of the ferruginous sandstone of southeastern 

 Missouri and southwestern Illinois there is not a single con- 

 tinuous stratum, but a large number of disconnected deposits, 

 lithologically indistinguishable, and lying, at least, at two very dif- 

 ferent geologic horizons. One horizon is below the Kaskaskia 

 limestone and the other above that great rock-mass. The one is 

 early Carboniferous; the other mid-Carboniferous. The lower 

 continuous terrane is known as the Aux Vases sandstone; the 



"Am. Jour. Sci. (3), Vol. XLVIII, p. 327, 1894. 





