STRATIGRAPHIC CLASSTFIOATTON DIASTROPHIC CRITERIA 437 



distribution of these elastics; and strongly eroding streams, rising in 

 unusually elevated Canadian highlands, probably flowed southward 

 across New York and Pennsylvania. Decided warping of the Appa- 

 lachian Valley and of the more interior areas are credited to this time. 

 Warping or gentle folding, involving earlier Ordovician slates in central 

 Virginia, probably occurred also in the median depression of Appalachia, 

 while stronger folding is shown in eastern New York and the New 

 England States. (See also imder Gradational criteria, page 468.) It 

 is to be added, further, that the significance of the great emergence at 

 this time is much increased if we grant the idea of rhythm in occurrence 

 and volume of diastrophic phenomena, because it intervened between the 

 two periods in which the average altitude of the continent was less and 

 the submergences greater than in any other period. 



The geographic conditions during the transition from the Ordovician 

 to the Silurian difl:er in an important feature from all the pre-Pennsyl- 

 vanian intersystemic stages ; namely, in that the earliest marine deposits 

 (Richmondian) of the succeeding period do not occur in the Appalachian 

 troughs but are found in the broad median area of the continent and 

 on its outer border (Anticosti). Indeed, the Silurian is the only Paleo- 

 zoic period that is not more completely represented by deposits in the 

 Appalachian region than in the Mississippi and Ohio valleys. From 

 all these facts it is clear that in the closing Ordovician and early Silurian 

 stages the whole Appalachian region was considerably elevated and 

 the middle and eastern parts of Appalachia itself most probably sub- 

 jected to profound orogenic movements. Really comparable conditions 

 occurred here in the Paleozoic only at the close of the Tennessean and 

 in the early Pennsylvanian, with this difference, that the western, rather 

 than the eastern part of Appalachia and the northern part of Llano 

 were greatly elevated, and in the course of their erosion supplied enor- 

 mous amounts of clastic material now included in the great Pottsvillian 

 formations in the Appalachian and Ouachita troughs. Both of these 

 periods of folding deserve high rank in the classification of orogenic 

 movements. 



As to the movements that crumpled the deposits in the Appalachian 

 Valley and that are usually credited with having given birth to the Ap- 

 palachian Mountains, the date of their occurrence is fairly open to ques- 

 tion. It is a long time since geologists have agreed that these movements 

 began late in the Pennsylvanian and were probably completed before or 

 early in the Triassic ; and this view is almost universally accepted today. 

 It is with no small degree of trepidation, therefore, that I request re- 

 consideration of the facts on which it is founded. Movements doubtless 



