GEOLOGIG\VERSUS LITHOLOGIE 235 
ous deposition, from the period represented by the Devonian 
fauna to that of the Lower Carboniferous. The writer then, 
for the present, accepts the local lithologic fact, and the 
absence of an observed explanation for the faunal change, as 
determining the treatment of the formation as a unit. If in 
the tracing out of the Ouray limestone it should be found that 
the inference of continuous sedimentation is incorrect and that 
a traceable and really sharp line, indicating a stratigraphic break, 
exists near the center of the apparent unit, the writer would favor 
an expression of that fact upon the map, if feasible. How that 
should be done is a matter of detail, from his standpoint. If the 
break in question should be found to represent a period of 
orogenic disturbance, with overlap of the Lower Carboniferous 
member, the desirability of expressing that fact wherever the 
limestone complex is found would be great. But for the region 
in which the apparent lithologic unit is now known it may be 
best that it should receive a single name, even if its lower and 
upper members be designated, respectively, as Devonian and 
Carboniferous. 
Another phase of the problem of establishing cartographic 
units may be illustrated by the stratigraphic section of the Rico 
Mountains, in southwestern Colorado, studied by a party in 
charge of the writer, which was cited by Mr. Willis as an 
example of unwarranted departure from the rule requiring that 
lithologic character alone should be used in discriminating 
‘“‘formations.”’ The section in question has a total thickness of 
about 4,500 feet, and lithologically presents a series of irregularly 
alternating conglomerates, sandstones, shales, limestones, marls, 
and all manner of transition beds, few of which exceed one hun- 
dred feet in thickness. There are probably fifty or sixty litho- 
logic divisions which might be established and traced for some 
distance, most of which would have a fair degree of lithologic 
uniformity. Only by such detailed mapping can the conditions 
of deposition be graphically expressed. Any grouping of unlike 
units obscures that expression. There is no ‘‘lithologic individ- 
* Loc. cit., p. 501. 
