148 IOWA ACADEMY OF 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. 



Fhi/sior/raphic Develoinnent. — 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 ihey 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. 

 + U. S. Geol. Sur., Mon. I, pp. 393-403, 1890. 



