48 Upham — Review of the Quaternary Era, 



Quaternary silt deposits, closely related to the loess, form 

 broad expanses in the Great Basin and in the San Joaquin val- 

 ley of California, attaining depths of at least 1,500 to 2,000 

 feet and more, as is known by artesian borings which at these 

 depths fail to reach the bed-rock. These deposits consist 

 mainly of fine-grained calcareous earth or clay, porous, gray to 

 yellow in color, which is used by the Indians and Mexicans for 

 the manufacture of sun-dried bricks, known by the Spanish 

 name " adobe." This name is also used for the earth from 

 which the bricks are made, and Russell has therefore adopted 

 it for this extensive geologic formation of the arid region. Its 

 broad flat tracts are bordered by steep mountain ranges, and 

 others rise from it like islands. The origin of the adobe, as 

 Russell has shown, is from the waste of the mountain slopes, 

 chiefly through the action of ephemeral rills and streams, born 

 of passing showers.* Though it is still increasing in thickness, 

 evidently far the greater part of this formation was deposited 

 before the humid epochs recorded by lakes Bonneville and 

 Lahontan, but after the early Quaternary orographic move- 

 ments which produced the grand topographic features of the 

 region. It belongs therefore mainly to the first Glacial epoch 

 and the ensuing long interglacial epoch. 



Russell has solved the difficult problem of the loess of China 

 by comparing it with the adobe ; and the great Quaternary 

 deposits of the Mississippi valley, and of the coastal plain of 

 the Southern States, find analogues in the basins of the Ama- 

 zon and La Plata, of the Rhine and the Po, of the .Nile, of the 

 Indus and Ganges, of the Yang-tse Kiang and Hwang Ho, and 

 of the Lena, Yenisei, and Obi. In particular, we may compare 

 the great Indo-Gangetic plain, which stretches across India 

 south of the Himalayas, with the Appomattox and Columbia 

 formations. This immense alluvial plain, the richest and most 

 populous portion of India, covers about 300,000 square miles, 

 rising from the sea level to an elevation exceeding 900 feet on 

 the water-shed between the Indus and the Ganges. Its pre- 

 vailing formation is fine silt or clay, more or less sandy, with 

 gravel near the borders of the plain. Its central portions have 

 a thickness of at least 400 to 700 feet, as determined by borings 

 which do not at these depths reach the bottom of the alluvium ; 

 and this entire deposit, according to Medlicott and Blanford, 

 is of Quaternary age and of fresh-water origin, having been 

 laid down by the flood stages of the rivers that descend from 

 the very rainy southern slopes of the Himalayan range. f 



The stratified or modified drift, gathered from the ice-sheet 

 by superglacial and subglacial rivers, and spread beyond its 



* Geol. Magazine, III, vol. vi, pp. 289-295 and 342-350, July and August, 1889. 

 \ Manual of the Geology of India. 1879, Part i, pp. 391-421. Quart. Jour. 

 Geol. Soc. London, vol. xix, 1863, pp. 321-354. 



