THE STRATTGRAPHIC COLUMN 379 



development, can give no adequate conception of the missing intervals; 

 and the physical evidence of discontinuities of sedimentation is even 

 more uncertain. An unconformity indicating absence of sediments of 

 four or five consecutive periods may be no more readily discernible than 

 another representing a relatively insignificant time interval. In fact, 

 the longer the interval of emergence the more complete the baseleveling, 

 and consequently the less conspicuous the contact irregularities. No 

 better illustrations of the principle can be found than those briefly men- 

 tioned in an earlier part of this paper in discussing the effects of currents 

 on deposition in continental seas and in describing certain overlaps (see 

 pages 305 to 311, 449 to 467, and 526 to 532). 



Additions to the Paleozoic column since 1850. — Since the Potsdam 

 sandstone was thought to be the oldest fossiliferous sedimentary rock 

 many thousands of feet of older fossiliferous deposits have been found 

 to which the name Cambrian is now almost universally applied. Even 

 yet this "Lost Interval" is far from being wholly accounted for, th!e 

 several intercalated stratigraphic units being in most cases separated 

 from each other by more or less clearly indicated gaps of undetermined 

 time values. Similarly, the Eopaleozoic ages following the Potsdam 

 have been expanded so that instead of an insignificant 200 feet of Cal- 

 ciferous between the Potsdam and the Chazy we know that elsewhere 

 in America this interval comprises deposits aggregating in thickness to 

 no less than 7,000 feet of limestone and dolomite. And the end is not 

 yet, for in the thickest sections of this interval we still encounter unde- 

 niable evidence of repeated interruption of sedimentation. 



The Ordovician expansions are scarcely less in value than those of 

 the older systems of the Eopaleozoic era. We have learned that the Saint 

 Peter sandstone represents the median to upper part of an unnamed 

 group or series of sediments of considerable though not yet accurately 

 determined time value, that followed the Canadian and preceded the 

 first of the Chazyan deposits. Then the Stones Eiver and Blount groups, 

 together comprising the Chazyan series, have been shown to have an 

 aggregate thickness in the A;ppalachian Valley more than four times 

 as great as that of the three divisions of the series in the Champlain 

 Valley. Finally, the lower Mohawkian Black Eiver group, which is 

 thin and very unequally distributed in New York, is now known to be 

 represented by deposits in east Tennessee and the Mississippi Valley, 

 having a total thickness of more than 1,000 feet. Even the upper 

 Mohawkian or Trenton group comprises a greater thickness of limestone 

 than was suspected twenty years ago. 



