88 J. Barrett — Upper Devonian Delta of the 



tant as the facts interpreted ; but those criteria which are used 

 by geologists to determine the original nature and limits of 

 eroded formations are less well demonstrated and are much 

 more vaguely employed than are those used for determining 

 the modes of origin of strata. In the study of the existing 

 sediments observation and demonstration are able to follow 

 fairly close to hypothesis, but in the more difficult study of that 

 which is no longer existent unchecked hypothesis still largely 

 prevails. This second part of this subject is taken up, there- 

 fore, in discussing the causes which have determined the 

 present limits of the Upper Devonian formations and the lack 

 of relationship between the present outcrops and the former 

 limits. 



It has been customary, on paleogeographic maps, to draw the 

 original limits of formations at no great distance beyond their 

 present outcrops. On the other hand, areas of ancient rocks 

 tend to become regarded, unconsciously to the thinker, as land 

 areas through all the younger ages. To a certain degree both 

 these principles as rules of guidance point in the direction of 

 the truth, and it is only by following them in a general way 

 that paleogeographic maps may be constructed. Yet in any 

 particular problem these principles may lead to large errors. 

 Sediments may originally have existed hundreds or thousands 

 of feet thick and reached hundreds of miles beyond their present 

 boundaries and now leave no trace. An extended discussion 

 of these subjects is required in connection with the Upper 

 Devonian, as is seen on comparing the present conclusions with 

 those of previous writers. This part, on the relation of present 

 outcrops to causes other than the original limits, is preparatory 

 for the more definite evidence which follows in the third part. 

 The preliminary discussion shows that there is no reason why 

 the Upper Devonian sediments may not have extended north- 

 ward to beyond Lake Ontario and eastward to the margin of 

 the present coastal plain. In the third part, to be published in 

 a following number, the indications given by the strata as to 

 their former extension are taken up and several independent 

 lines o f evidence converge to the conclusion that the Upper 

 Devonian did extend originally to the neighborhood of these 

 limits. 



Present Extent and Thickness of the Upper Devonian. 



The map of the Appalachian geosyncline, fig. 1,* shows the 

 limits of the Upper Devonian outcrops and the contours give 

 approximately the total thickness of sediments which were 

 deposited during the Upper Devonian. 



* Figs. 1 to 4 inclusive are published in Part I, this Journal (4), xxxvi, pp. 

 429-472, 1913. Fig. 1, for convenience of reference is republished in this 

 part also. 



