554 E. O. ULRICH EEVISION OF THE PALEOZOIC SYSTEMS 



and the argument fixing the stratigraphic position of the Saint Peter 

 series necessarily too long and many-sided to be set forth at this time. 

 For present purposes it will suffice to say that the problem has been very 

 carefully studied and the conclusion reached that the group or series of 

 formations beginning with the Everton limestone and ending with the 

 Joachim is older than the Stones Eiver and younger than the last of the 

 long sequence of Canadian deposits in central Pennsylvania. Assuming 

 the validity of this conclusion, very extensive emergence at the close of 

 the Canadian is established. The only reason that causes me to hesitate 

 in declaring that this emergence affected the whole continent is my 

 present inability to decide whether certain graptolite shales at Summit, 

 Nevada, fall within this emergent stage or whether they are younger 

 or older. (See page 676.) 



(17) The principle of maximum thickness of overlapping forma- 

 tions. — The maximum thickness of an overlapping formation whose 

 distal edge is overlapped from another or perhaps opposite direction by 

 a similar thin wedge (see figure 8) is thought to have been deposited 

 before the land transgression of the second began. Essentially the same 

 principle is involved when two overlapping formations which invaded a 

 given area from the same oceanic basin are directly superposed. In such 

 cases it is thought that the hiatus between the two formations may, as a 

 rule, be interpreted as including one or more intervals in which a cor- 

 responding number of invasions from other quarters occurred that failed 

 to reach the locality in question. 



This principle is based on the hypothesis of land tilting which, whether 

 of continental or geographically minor extent, assumes that the attitude 

 of the continent with respect to sealevel, or of such parts of it as were 

 affected by the successive overlaps, must have prohibited submergence of, 

 say, the southern part of North America while sea invasion was progress- 

 ing in the north. The hypothesis assumes further that the reversal of 

 the prevailing general or local tilt was accomplished in the interforma- 

 tional intervals during which marine deposition was confined to epicon- 

 tinental or extracontinental areas now inaccessible. Eemembering that 

 marine deposition requires preceding submergence, apparent exceptions 

 to the rule may be explained by assuming that at such times the reversal 

 of the tilt was insufficient to effect submergence. The absence of Cin- 

 cinnatian, Silurian and Devonian deposits on the east side of the Nash- 

 ville dome is explained in this manner on page 425. 



Demonstration of the principle by the Lowville overlap. — Though the 

 (demonstration of the proposition often rests almost entirely on correlation 



