REPRESENTATION OF GEOLOGICAL FORMATIONS 699 



case of the St. Clair limestones of Arkansas, already referred to, 

 and the formations of the Rico mountains, mentioned by Mr. 

 Cross, are good examples in which strict lithologic separation 

 was not obvious at first glance. In reality, Cross and Spencer's 

 " Hermosa," "Rico," and "Dolores" formations approach 

 definite geological formations only approximately. They repre- 

 sent no nearer the real geological formation than do purely litho- 

 logic individuals. Exact faunal studies in the Rico and neigh- 

 boring districts are likely to require the divisional lines to be 

 drawn at quite different horizons. When the fuller geological 

 history of the region shall have been made out further rectifica- 

 tion will doubtless be found necessary. 



As a matter of fact, the lithologic individual based primarily 

 upon lithology and secondarily upon fossils contained, and the 

 "geological division" based upon the fossil characters have 

 essentially the same kind of values as geological elements. But 

 Mr. Cross, in his delimitations, does not depend entirely upon 

 the faunal features, for he goes on to reenforce his statements 

 by giving other reasons for drawing his lines where he does. 

 Lithologic features are manifestly among the most important 

 criteria in tracing the "formations." In the same way it is quite 

 apparent that the advocates of strictly lithologic individuals in 

 mapping do not and cannot depend wholly upon a uniform 

 rock character. 



This phase of the question leads to the statement of a more 

 general one, that the main thing is to give clearer definition, 

 than has usually been done, of each cartographic unit proposed. 

 The unit should be defined according to (1) geographic distri- 

 bution, (2) topographic expression, (3) lithologic nature, (4) 

 stratigraphic delimitation, (5) biotic definition, and (6) mineral 

 content. 



When this shall have been done the foundation will have 

 been laid for the establishment of real Geological Formations 

 expressive of the geological history of the area mapped. The 

 approximate "lithologic" map will not have to be materially 

 changed, but only accompanied by a few words of additional 

 explanation. Charles R. Keyes. 



