86 The Ohio Journal of Science [Vol. XVII, No. 3, 



in the succession of rock layers indicate breaks in the process of 

 sedimentation. Such a break is called an unconformity. 

 The size of the area over which the unconformity extends and 

 the amount of the break are used in determining the rank of the 

 unconformity, i. e., whether it shall be used simply to mark the 

 limits of two formations, or to mark the boundary between 

 two systems or groups of rocks, or two periods or eras of time. 

 Just as birthdays count off years so local and slight breaks or 

 changes in the sedimentation mark off short time units; and just 

 as centennial and millennial celebrations mark oft' hundreds and 

 thousands of years, so widespread unconformities and great 

 breaks in the geologic record mark off the larger periods and 

 even larger eras of time. 



In recent years unconformities of considerable significance 

 have been found where they have been passed over unnoticed 

 before. Perhaps others are yet to be found; and probably some 

 now known only locally will be found to be very extensive. 

 It is only by the careful study of hundreds or thousands of 

 exposures over broad areas and the most judicious correlation 

 from place to place that the facts of the extent and the amount 

 of a given break can be known. Here then, is ample reason 

 for the most detailed stratigraphic work over all known lands. 



If enough were known of the unconformities and the char- 

 acter of each layer of rock wherever it occurs, it might be 

 possible to plot on a map the distribution of lands and seas at 

 any given time. Such mapping and the interpretation that 

 goes with it is called paleogeography. It has been but a few 

 years that men have had sufficient data to attempt such map- 

 ping. The first maps were supposed to give the extent of seas, 

 bays, gulfs, and land areas for a whole period. They were 

 in the same order of accuracy as a historical map of Europe 

 which should attempt to show on a single map the distribution 

 of the nations through the whole Christian era. In Europe in 

 1915-16 it is necessary to date the map to the month and the day 

 to make it right. Paleogeography cannot be considered far 

 advanced until it can produce one map for each of a hundred or 

 more dates throughout the earth's sedimentary history, not 

 only for one continent, but for six, with some intercontinental 

 connections. In order to make such maps it will be necessary 

 to have an enormous body of data for the area and for each 

 particular horizon mapped. We cannot hope for perfect 



