604 C. SCHUCHERT PALEOGEOGRAPHY OF XORTH AMERICA 



the Siluric should close with the Manlius or with the Oriskany. During 

 this entire time there was very little inundation, the small seas were oscil- 

 lator} T , and there was no decided positive or negative movement of the 

 strand-line. An analysis of the upper curve, however, shows that the 

 Manlius was the last formation of the Siluric, being the highest point of 

 the emergence, and that while the strand-line during the Helderbergian 

 and Oriskanian was positive, it was not markedly so until the Onondaga. 

 Taking all the facts into consideration, however, this upper curve proves 

 to be in harmony with the later views of the majority of European and 

 American stratigraphers. The writer therefore regards the nodal point 

 on this upper curve as the best expression of the more natural division 

 between the Siluric and Devonic. 



During the past ten years the dividing line between the Devonic and 

 the Mississippi has also been debatable in America, but the two curves 

 of the chart, when considered in combination, unfortunately furnish no 

 definite answer to the problem. The difference between the two curves 

 is due to a lack of detailed knowledge as to the areal distribution of the 

 Chemung and Bradfordian equivalents. The lower curve is thought to 

 more nearly represent the truth than the upper one, in which the deposits 

 of Bradfordian time fall into the Mississippic period. According to the 

 upper curve, however, this epoch is Devonic, but not much dependence 

 should be placed on this portion of the curve, for as yet this interval is 

 practically unknown in the Cordilleran sea. Disregarding the minor ups 

 and downs of the two curves, it may be seen that all the deposits from the 

 base of the Helderbergian to the close of the Keokuk belong to one great 

 diastrophic cycle. In the end this view may be found to be the correct 

 one, but until more of the paleogeography of the world is deciphered noth- 

 ing decisive can be said at this time. Should this view finally prevail, the 

 term Mississippian will even then be found useful. 



The new period Tennesseic is borne out by both curves, but is espe- 

 cially emphasized by the upper one. Xeither curve, however, is thought 

 to be expressive of the actual amount of inundation, because knowledge 

 of the areal distribution of these deposits is as yet not exact. In any 

 event, enough is known of these formations, of their faunas, and the dias- 

 trophism indicated by their areal spread, to denote a general movement, 

 both negative and positive, of the strand-line. 



Beginning with the Pottsvillian, the strand-line became decidedly 

 positive throughout America, with its maximum at the close of this 

 epoch. Then very slowly the tide turned, and there was continuous emer- 

 gence to the close of the Permic. On this basis alone there is no 



