150 IOWA ACADEMY OF SCIENCES. 



which are still geotectonically mountains, though they have 

 been completely base-leveled, and have been long since buried 

 beneath later sediments. With these old mountains the 

 cycles of orogenic development are properly regarded as 

 beginning at the time when the strata were compressed, and as 

 extending through the periods when they were bowed up, then 

 planed off nearly to sea level, and submerged perhaps, until 

 degradational products were deposited upon their upturned 

 edges. The record of the completed cycle of mountain- making 

 is the measure of orotaxial chronology. The division planes 

 cutting the geological column into systems, series, or smaller 

 groups are, theoretically as well as actually, the lines of 

 unconformities. In the case of the more extensive, they no 

 doubt represent base- leveled surfaces or peneplains 



In all cases, great or small, the erosion plane and period of 

 degradation of the land has its equivalent in the sea, in an 

 accumulation of sediments. An ancient plane of unconformity, 

 as it is now open to observation, may pass gradually into a 

 great plane of sedimentation. In the grander unconformities, 

 in which the plain of discordant sedimentation represents 

 essentially an old peneplain, the corresponding stratum which 

 was deposited in the sea area is usually a limestone. In fact, 

 most limestone formations are now looked upon as represent- 

 ing deposition during periods when the land adjoining was a 

 graded surface, or a plain of faint relief lying but little above 

 sea level This being the case, all unconformities have much 

 greater significance than heretofore suspected. 



These surfaces of unconformity and their representative 

 great planes of sedimentation are the only absolute datum 

 planes from which the measurement of formations can be 

 estimated. Theoretically the formation is generally con- 

 sidered as a fixed and clearly defined unit; in practice it is 

 found to be ill-defined and incapable of definition in any but 

 the vaguest terms. But from the datum plane of the uncon- 

 formity a new sequence of strata begins, sharply and clearly 

 set off from the formations below. Many, and perhaps most, 

 of the sharp lines of divisions are now effaced over much of 

 the existing land surface, but in this respect the record is not 

 more imperfect than any other, for the formations themselves 

 have been swept away. The longer a land area has remained 

 above sea level, the greater is the liability of the records of 

 the earlier events being lost. Over other districts in which 



