THE PLIOCENE PERIOD. 307 



toward the source of sediment supply, were never mantled by the 

 formation. 



As set forth in Volume I, the overloading of streams is greatly 

 affected by the mode of precipitation and the vegetal covering of the 

 region. Diversifying agencies, particularly when attended by sub- 

 aridity, tend toward concentrated precipitation, which greatly acceler- 

 ates erosion. A change of vegetal covering, generally involving a 

 decrease in the amount of protection, usually accompanies a climatic 

 movement toward diversity and aridity, particularly if a reduction 

 of temperature attends the change. All these abetting agencies are 

 assignable with good reason to the Pliocene movement, not only on 

 general grounds, but on the specific implications of this formation, 

 as already indicated. 



The erosion and re-deposition of material once deposited in the 

 manner sketched above, is regarded as an important feature, and the 

 source of grave difficulty in the correlation of the formation and its 

 derivatives. The erosion and re-deposition of the material during 

 the deposition of the main formation did not cease there, but has 

 been in progress to recent times, and the series of derivatives so closely 

 resemble the parent formation in structure and material that their 

 reference to their proper stages is exceptionally difficult. The close 

 resemblance of the derivative deposits to the parent formation in 

 structural features throws light on the mode of original deposition, for 

 in some cases the later method is certainly known. 



If it shall ultimately be shown that the seaward portions of the 

 Lafayette, now concealed or unstudied, are marine, the preceding hypo- 

 thesis would need to be modified only by supposing that as the feeding 

 ground of the streams was bowed up, the coastal border of the plain 

 was submerged. In this case, there should have been estuarine for- 

 mations in the seaward valleys. 



The chief alternative view relative to the origin of this strongly 

 characterized formation assigns it to marine deposition 1 during a 

 stage of submergence essentially co-extensive with the area of the for- 

 mation. This hypothesis has been faithfully applied by geologists 

 of wide familiarity with the phenomena and abandoned as untenable 

 even where the conditions seem most to favor it. It is, however, still 



1 McGee, 12th Ann. Rept. U. S. Geol. Surv. 



