148 
IOWA ACADEMY OP SCIENCES. 
It begins to show itself in the adoption of more than one 
method of correlation. In its latest significance, the term has 
come to cover the united conclusions derived from all methods 
of correlation. At first glance the method has much merit; 
but further consideration brings out the same fatal defects, in 
its application to general problems, that are apparent in the 
older and more widely used methods. It is essentially local in 
its extension, and hence is on the same plane as the individual 
methods it brings together. It fails to parallel the strata of 
provinces of different geological origin. 
PhysiograpUic Development . — The modern physiographic prin- 
ciples, as enumerated by Davis, Gilbert f and others, have an 
important bearing upon geological correlation. Their direct 
application, however, is confined to only the later formations. 
Their chief value lies in the suggestions they have made 
regarding the real basis of geological classifications and corre- 
lations, and in showing conclusively that a general considera- 
tion of the problems is not to be sought in any one of the 
criteria yet set forth. The fundamental principle that is of 
such prime importance to stratigraphical geology is that with 
each marked uprising of the land surface there are produced 
phenomena which are as ineffaceably impressed upon the 
portion of the earth’s crust above the sea, as is deposition 
itself below the water level. The final reduction, through 
erosion of the elevated land surface, to a more or less even 
plain lying but little above the sea, the formation of a pene- 
plain, is a phase in the geological development of the region, 
the full force of which has been until recently entirely over- 
looked. When the lowland plain is depressed below tide level 
and covered by sediments, unconformable relations of the two 
formations are produced, but the line of unconformity, instead 
of indicating merely an hiatus, or blank gap, devoid of inter- 
est, represents a chapter in the history of the region that is 
even more pregnant of eventful happenings than those 
recorded by the contiguous formations that were formed 
during the same period. The time-gap, and not the forma- 
tions, are, therefore, the all-important features in marking off 
the ages, epochs and periods of geological history. The latter 
stand for continuity of record; the former for interruptions 
which render a classification possible. 
*Nat. Geog*. Mag., Vol. I, pp. 183-253, 1889. 
t U. S. Geol. Sur., Mon. I, pp. 393-402, 1890. 
