DAVIS. — ROCKY MOUNTAIN TERTIARIES. 361 



sheets of gravels, sands, or clays, the coarser sediments frequently show- 

 ing cross-bedding and local unconformities, the finer sediments generally 

 possessing an even stratification. The area over which such deposits may 

 accumulate is shown by existing fluviatile plains to rival that of the 

 western Tertiary deposits. In a region of considerable precipitation, 

 with a background of mountains from which abundant waste is shed to 

 lowlands in the foreground where the rivers have free discharge to the 

 sea, the activity of fluviatile aggradation is often equal to the average 

 activity of the deforming forces that tend to cause marine submergence 

 or to produce broad lake basins. A slight acceleration of littoral 

 depression might cause submergence, or a rapid local warping might pro- 

 duce a lake ; a pause iu these movements would allow the rivers to 

 convert the sea border or the lake into a fluviatile plain again. The 

 preponderance of one condition or the other might be determined by the 

 proportion of fine, evenly stratified layers (if such deposits are necessarily 

 marine or lacustrine, and out of the reach of river action) to variable 

 strata with cross-bedding and local unconformities in the resulting' 

 deposits, as well as by a study of the fossils that they contain. 



The capacity of rivers to form extensive deposits of fine texture and 

 even stratification seems in particular to be underrated. It is true that 

 a torrential river, gathering coarse detritus and exposed to heavy floods 

 in its headwaters among lofty mountains, may carry cobbles and pebbles 

 many miles forward upon a piedmont fluviatile plain. The artificial 

 enclosure of its channel by dikgs to prevent overflow probably increases 

 the distance to which pebbles can be carried, as on the plain of the Po ; 

 but if the river is free to spread upon an aggrading surface, the pebbles 

 would be sooner laid down. In arid regions the coarse piedmont depos- 

 its assume great importance, as is more fully stated below. On the other 

 hand, rivers of moderate size, rising in uplands of moderate height, may 

 contribute chiefly very fine and well stratified sands and clays to the 

 plains that they aggrade. This will be especially true if their headwaters 

 drain regions of deep soils, such as occur on slightly elevated peneplains ; 

 or of weak strata, such as are found in basin deposits of earlier date. At 

 times of high water and overflow, rivers of this kind will spread layers 

 of fine silt far and wide over their plains, and the repetition of this pro- 

 cess must lead to the formation of thick deposits, fine in texture and even 

 in structure, with little admixture of coarser sands and pebbles. Ripple 

 marks, foot-prints, and raindrops may be preserved in the sediments of 

 shallow flood-plain lagoons, and mud-cracks may form as the lagoons are 

 dried up. The plains of the Po and of the Ganges, and the great fan of 



