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 
