228 WHITMAN CROSS 
recorded in the area covered as is practicable. To that end its 
cartographic units must be established with regard to ad/ the 
facts and conditions of the case, and not upon the restricted 
basis of any part of those facts. Of course this discussion does 
not refer to the mechanical limitations of map-making, nor to 
local maps of large scale designed for some special purpose, but 
to the problem involved in the mapping of a geologic province 
on a scale similar to that of the Atas of the United States Geologi- 
cal Survey. 
This idea of the geological map is based upon the considera- 
tion that geology is the broad science of the earth, viewing it as 
originating in a manner still a matter of hypothesis, as develop- 
ing through a long succession of changes, from that uncertain 
state to the present conditions. Geology deals with all the pro- 
cesses involved, their modes of operation, and the results. 
Development implies a succession of events, a series of changes, 
a history, and so some conception of geological chronology is 
inseparable from the discussion of rock masses as geologic forma- 
tions. The available record of earth history prior to the pres- 
ent time is in the rock masses and their relations to each other. 
Geological -mapping, therefore necessarily consists chiefly in 
representing the distribution of rock masses. The map will ful- 
fill the purpose of expressing geologic development in pro- 
portion as the units of representation are discriminated with 
reference to all the record contained in them. If the cartographic 
units are determined by consideration of only a part of the 
information which may be used, the map will be restricted, and 
unnecessarily so. 
It seems to the writer that the proposition to map ‘‘litho- 
logic individuals,” and to call the result even by courtesy a 
geological map, ignores very largely the fundamental distinction 
between the vock, as an object of certain petrographic character, 
and the geologic formation, a mass of importance in the crust of 
the earth, and containing far more than the record of physical 
conditions controlling its formation. The separation of petrog- 
raphy from structural geology was made nearly one hundred 
