GEOLOGICAL MAPPING 1113 



being hence conducted on a lit'nic basis. "As uniform conditions 

 of deposition were local as well as temporary it is to be assumed 

 that each formation is limited in horizontal extent. The forma- 

 tion should be recognized and should be called by the same name 

 as far as it can be traced and identified by means of its lithologic 

 character, its stratigraphic association and its contained fossils." 

 (i8:7J.) In mapping it is often impossible to draw a sharp line 

 when two contiguous formations grade into each other. In such 

 cases the boundary has to be more or less arbitrarily established. 

 An example of this is the Siluro-Devonic boundary of the Helder- 

 bergs. Here in some places the Manlius or uppermost Siluric 

 member is found to grade up into the Coeymans or lower Devonic 

 member both lithically and faunally.* 



mapping on Faunal Basis. When two formations of the same 

 lithic character are separable by their faunal content only it is 

 often found practicable to map them separately on a purely faunal 

 basis. In such a case it is frequently necessary to represent the 

 transition portion by a commingling of colors of the two series. 

 Sometimes the faunal change is a horizontal one, where two distinct 

 faunas occupied different portions of the province at the same time, 

 there being no change in lithic character. An example of this is 

 seen in the two Portage faunas of New York, the Ithaca and the 

 Naples, which existed side by side throughout Portage time. This 

 is expressed on the map by two colors, or two shades of the same 

 color, which horizontally pass into each other or overlap along the 

 line of interlocking of the faunas. (Clarke-2.) 



Mapping of Discontinuous Formations. It is a matter of com- 

 mon experience that formations change in passing away from the 

 shore line, certain more terrigenous ones (as sandstones, etc.) 

 coming to an end and others of more truly marine origin (such as 

 limestones) appearing. As a result, detailed maps of adjoining 

 areas, not parallel to the old shore line, may exhibit considerable 

 diversity of formations, and it may even happen that quadrangles 

 not so far removed from each other may exhibit scarcely any for- 

 mations of the same name. Thus the Columbia quadrangle of 

 Central Tennessee (Hayes and L^lrich-9) and the McMinnville 

 quadrangle of eastern Tennessee (Hayes-8) have no formations 

 in common, though they are separated by an interval f of only 



* This is not always the case, however, and this close relationship has been 

 denied by Ulrich. But there can be no question of this gradation in the Schoharie 

 region of New York (see Grabau-6). 



t The Chattanooga formation which appears on both maps is not of the 

 same age, being younger in the more eastern quadrangle. 



