GEOLOGIC VERSUS LITHOLOGIC 225 
articles, undoubtedly obscure the meaning of their discussions. It 
is true that their usages are in accordance with the cited defini- 
tions of the United States Geological Survey regarding carto- 
graphic units, and that the usage is now becoming common, so 
that no special criticism of their course can be intended in this 
reference. But as will appear, the writer’s conception of the 
geologic formation is of something which does not receive 
recognition in the articles by Messrs. Willis and Eckel, while 
we are all nominally discussing the same thing, namely: the 
desirable and practicable scope of the geological map and the 
nature of the problem presented to the geologist in making such 
a map. In all frankness the writer is unable to decide to what 
extent the conception of the geologic formation entertained by 
him is actually coincident with that of the nominally restricted 
“formation”’ or “lithologic individual” in the minds of Messrs. 
Willis and Eckel, although it is clear that, in spite of the defini- 
tions they profess to follow, some other factors than those of the 
definitions are really there. 
Mr. Eckel entitles his article, ‘‘ The Formation as the Basis 
for Geologic Mapping.” Under the restricted technical use of 
the word he soon states that he means “ lithologic unit,’ some- 
thing discriminated and defined solely on lithologic characters. 
But in the discussions of Mr. Eckel and Mr. Willis, and of other 
writers professedly dealing with a formation thus defined, there 
is continual evidence that to some extent other factors enter 
J 
into their idea of what they designate a ‘‘lithologic unit,” such 
as stratigraphic continuity, even if accompanied by change of 
lithologic character. The terms ‘ lithologic individual” or 
“unit” used in defining formation, or givenas practically synony- 
mous with it, refer in their common or apparent meaning to a 
rock, one of the units treated by the systematic science of petrog- 
raphy. Mr. Willis constantly refers to the mapping of rocks. 
The definition of the ‘‘ formation” as a “ lithologic individual ” 
lays stress on the idea that it is a result of continuity of physical 
conditions, securing to the sandstone or limestone mass its 
unity of lithologic character. But in the acknowledgment of 
