FORMATION THE BASIS FOR GEOLOGIC MAPPING 71 1 



in favor of the latter unit ; for tlie relation existing between 

 time taken in deposition and thickness of formation is much 

 more traceable than that between extent of variation and time 

 taken in evolution. 



In considering the adoption of a basis for geologic mapping 

 the biologic unit has, therefore, no antecedent claim to special 

 consideration on scientific grounds ; and its value can be com- 

 pared directly with that of the lithologic unit in regard to their 

 relative practicability and utility. 



THE RELATIVE PRACTICABILITY OF THE TWO SYSTEMS 



A lithologic unit can usually be so described and defined as 

 to be readily identified by any future worker in the same or an 

 adjoining area through which the formation extends. 



It should also be noted that all geologists substantially agree 

 upon the use of the terms in which lithologic individuals are 

 defined, and will consequently agree closely in their valuation 

 of any series. "Limestone," "sandstone," "slate," "conglom- 

 erate," all have fairly definite meanings, and it is improbable 

 that terms such as these are used in very different senses by 

 different geologists. The biologic unit, on the other hand, is 

 rarely capable of being so described or defined as to be accepta- 

 ble to all paleontologists. The difference is that the lithologic 

 individual is a fact; the biotic individual, as commonly described, 

 is a fact plus an interpretation ; and while there may be sub- 

 stantial agreement as to the facts in any given case, it is but 

 rarely that the interpretations will coincide. 



The Hudson formation in the vicinity of Albany, N. Y., pre- 

 sents an excellent example of some of the practical difficulties 

 encountered in attempting to represent faunal distinctions on a 

 map ; and a brief sketch of the conditions may be of interest in 

 the present connection. 



The "Hudson River group" of the earlier classifications — 

 the "Hudson formation" of the present system of nomenclature 

 — comprises in the central portion of the Hudson River valley, 

 where it is best developed and exhibited, a thick and extensive 



