GEOLOGIC VERS SS SEITHOLOGI € 227, 
they must also conceive of the geological map as more than 
a map of lithologic individuals could be. Mr. Willis states, with 
justice, that we are often perplexed to know what the divisions 
or cartographic units of a map are intended to represent.t. That 
is particularly true of formations purporting to be ‘‘lithologic 
individuals,” which are actually heterogeneous in lithologic char- 
acter and do not represent continuity in conditions of deposi- 
tion. The writer believes that every definition of a geologic 
formation chosen as a cartographic unit should contain a state- 
ment of all the factors determining and justifying its discrimina- 
tion, with due emphasis on the most important features of each 
case. 
Mr. Willis addressed himself in the cited article to the ques- 
tion ‘Should geologists map the record of physical conditions 
or the record of biological conditions—rocks or fossils?”’ and 
his answer was: ‘‘Both, but with distinctions.” The question 
might be assumed to be broad enough to cover the problem of 
the geological map if the term ‘physical conditions”’ could be 
interpreted as.referring to the general physical development of 
the earth as recorded in rock masses. But Mr. Willis’s whole dis- 
cussion is directed to maintaining the idea that geology is in 
such an elementary state of development that our present object 
must be restricted to mapping ‘‘lithologic individuals, while our 
associates, the paleontologists, distinguish the faunal units of 
stratigraphy.’ ? ‘The record of physical conditions”’ must then 
be regarded as referring only to the conditions causing the lth- 
ologic characters, by which the so-called “lithologic individuals”’ 
are to be discriminated. As already stated, the writer believes 
that geologists should map geologic formations, not rocks or fos- 
sils. A map of lithologic individuals, discriminated on the 
restricted grounds advocated by Mr. Willis, is not entitled to be 
called a geological map; it is a /zthological map, pure and simple. 
The question under discussion is really, ‘‘What should a geo- 
logical map represent?” The writer’s answer is that it should 
represent as much of the geologic development of the earth 
t Loc. cit., Pp. 557. 2 Loc. cit., p. 569. 
