380 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



The additions to the post-Ordovician parts of the stratigraphic column 

 in the past fifty years have been less than in the older rocks. Obviously, 

 the reason for this is that the fullest records of the older deposits are 

 found in regions that have been comparatively difficult of access and 

 in which, moreover, the stratigraphic relations of the several beds are 

 often much obscured by complicated structure. Still, important accessi- 

 ble additions to the Silurian, Mississippian, and Tennessean systems have 

 been made. The expansion of Silurian time we owe chiefly to refined 

 correlations of post-Niagaran deposits in New York, Maryland, Ohio, 

 and Michigan by Clarke, Schuchert, Hartnagel, and Grabau in the past 

 fifteen years. Through their efforts the aggregate thickness of deposits 

 in Am^erican continental seas between the top of the Guelph and the 

 top of the Manlius has been more than doubled. Eevision of the evidence 

 bearing on the time relations of the Niagaran formations will necessitate 

 further expansion of the Silurian by showing that nearly the whole of 

 the dolomitic limestones of this age in Wisconsin belong above the horizon 

 of the Rochester shale of New York. Finally, the reference of the 

 Richmondian deposits to the base of the Silurian does not expand this 

 system merely at the expense of the Ordovician, but actual expansion 

 of the Richmondian interval itself has occurred through study of deposits 

 of this age in the Mississippi Valley and on the Island of Anticosti. 



Very little has been added to the Devonian except by transference of 

 the Helderbergian from the Silurian and in the way of greater thickness 

 of some of the later deposits in the Appalachian Valley than had been 

 known before. However, a greater length of Devonian time than is 

 indicated by the relatively clastic character of a large proportion of the 

 sediments of this period in southeastern America is suggested by the 

 reported extraordinary thickness (6,000 feet) of limestone in Nevada 

 referred by Walcott and others to this system. Though some of this 

 great mass may not be of Devonian age, it yet seems certain that the 

 deposition of the remainder required sufficient time to insure apparently 

 greater than average time value for Devonian deposition in continental 

 basins. 



The great duration of the Waverlyan, Tennessean, and Pennsylvanian 

 periods could not be appreciated in the early days when our knowledge 

 of these ages rested almost entirely on the stratigraphic record found 

 in the Mississippi Valley and when the whole of this was comprised in 

 a single Carboniferous system. The more complete records subsequently 

 determined in the Appalachian region added much and in a manner 

 prepared us for the proof of the enormous time involved in these periods 

 that investigations in the far west has brought out. The aggregate 



