GEOLOGIC WVERSOS LITHOLOGIC 233 
clear and it may be found in association with comparatively 
unaltered portions of the original mass and exhibiting transition 
stages. But the great majority of these interesting records of 
earth history belong to the oldest periods of geologic time of 
which we have evidence in the rocks, and the nature of the 
primary mass can never be established. The discrimination and 
representation of metamorphic rock formations should, how- 
ever, be intended to express as much as practicable of this phase 
of earth development. 
The belief that a geological map of today must be limited to 
the exhibition of the distribution of local ‘‘lithologic individuals 
or units,’ seems to the writer not only inconsistent with the true 
conception of the object of the geological map but to ignore the 
known facts of the stratigraphic column. It is now desired to 
consider a few of the practical problems presented to the areal 
geologist in the preparation of a geological map. 
In the first place, it will be assumed that the terms ‘‘litho- 
logic unit” or ‘individual’ and the proviso that ‘they shall 
contain between their upper and lower limits either rock of uni- 
form character or rock uniformly varied in character,” imply 
that this unity in character has been derived from actual conti- 
nuity or regular oscillation of conditions of sedimentation during 
the epoch in which the formation was deposited. If such impli- 
cation is not intended, it may be remarked that a map of such units 
would so effectually obscure geological conditions that it could 
not be included within the scope of the present discussion. But 
the writer wishes to assert his belief, on the one hand, that many 
extensive sections of the stratigraphic column cannot be com- 
pletely divided into mapable units of the significance above 
implied, and on the other hand, that the inference of continuity 
of conditions, suggested by lithologic character alone, must be 
tested by all available facts of the case to avoid falling into 
serious error. This belief is based upon the writer’s personal 
d 
experience in the stratigraphic section of the Rocky Mountain 
province, ranging from the Algonkian to the Tertiary, and upon 
