300 GEOLOGY. 



some situations the gravels have been shifted repeatedly, always farther 

 from the mountains and to lower levels, with the result that they 

 now constitute a series of deposits, of somewhat different ages, rather 

 than a single formation which can be assigned to a definite epoch. 



Here are to be classed, probably, the Nussbaum formation of Colorado, 1 

 and equivalent but unnamed bodies of gravel in Wyoming, Montana, and 

 New Mexico, and, farther from the mountains, the Goodnight beds of Texas, 

 unconformable on the Loup Fork, the Uvalde 2 and Blanco formations of the 

 same state, the latter consisting of sands, clays, diatomaccous earths and some 

 limestone 3 (Reynosa, non-marine), as well as gravel. Gravels of similar age 

 occur in Kansas' (often cemented into "mortar beds") and western Ne- 

 braska 5 (Ogalalla formation). 



Formations of this class have been even less well recognized in the 

 Old World, but from the descriptions of the Indian geologists, it seems 

 probable that the great Siwalik formation, a derivative from the Hima- 

 layas in their rising stage, belongs to this class. The enormous and 

 abrupt elevation of the Himalayas, in close juxtaposition to the great 

 Indo-Ganges plain, presented extraordinarily favorable conditions for 

 such a foot-plain deposit, and the Siwalik formation may come to 

 be the classic example of aggradational deposition. 



The juxtaposition of precipitous heights and flat plains is not 

 the sole condition for aggradational formations. A less sharp differ- 

 entiation between feeding and lodgment grounds will suffice, when 

 adjustments are favorable. 



In the Mississippi basin, far from the Rocky mountains on the 

 west and the Appalachians on the east, there are patches of gravel on 

 various hills and ridges, which are interpreted as the dissevered rem- 

 nants of a once more or less continuous mantle of gravel and other 

 river detritus. Data are not at hand for the definite correlation of 

 these gravels, and they may not all be of the same age. They are 

 not older than late Cretaceous, and are older than the glacial drift. 

 The source of this material, which is almost wholly quartz, quart zite, 

 and chert, is partly local, but apparently more largely from the north. 



1 Walsenburg, Spanish Peaks and Pueblo folios, U. S. Geol. Surv. 



2 Vaughan, Uvalde folio, U. S. Geol. Surv. 



8 Penrose, 1st Ann. Rept. Geol. Surv. of Tex., 1890, p. G3, and Dumble, Jour, oi 

 Geol., Vol. II, p. 5G2. 



4 Haworth, Geol. Surv. of the Univ. of Kans., Vol. II. 



5 Darton, 19th Ann. Rept. U. S. Geol. Surv., Pt. IV. 



