356 TJic American Geologist. December, i89h 
a duration of probably one or two million years, which would 
permit the valley to have its origin chiefly by rock solution, 
as the deepening of the excavation to 200 or 250 feet would 
be at an average rate of no more than an eighth or a third 
of an inch for each century. 
When the more rainy and snowy climate of the Glacial 
period caused larger floods of the streams, especially during 
the spring months, the residuary loam and gravel mantling 
all the surface were subjected to exceptional denudation. 
Considerable material was swept down every stream course 
and ravine, until, on debouching into the main valley, the 
flood could expand more widely and so lose its velocity ar^d 
transporting capacity. The gravel and sand were then de- 
posited along the border of the broad bottomland and in al- 
luvial fans upon the lower part of the inclosing slopes 
wherever tributary streams or the fills of rains and snow- 
melting descended. 
Instead of forming continuous, level-topped terraces, at 
successive hights, up to nearly equal vertical limits on each 
side of the valley, like the terraces of modified drift on the 
Connecticut river along its upper half where it is the boun- 
dary between New Hampshire and Vermont, or like the usual- 
ly less numerous terraces of this kind in most valleys of 
glaciated countries, the Somme gravel and sand are of less 
amount and have gentle or steep slopes toward the center 
of the valley, presenting a terrace escarpment only where 
Emile Cartailhac, Mat^riaux, 1865 and onwird; La France Prehis- 
torique, 1889. 
J. Ladriere, Annales de la Societe Geologique du Nord, vols. 
XVIII-XX (summarized in Geikie's Great Ice Age, third ed., 1894, 
pp. 629-637). 
Albert Falsan, La Periode Glaciaire etudiee principalement en 
France et en Suisse. 1889. 
Prof. Daniel Wilson, Prehistoric Man: Researches into the Origin 
of Civilisation in the Old and the New World. 1862. 1865. 1876. 
James C. Southall, The Recent Origin of Man, as illustrated by 
Geology and the Modern Science of Prehistoric Archseology, 1875. 
Prof. Alexander Winchell, Preadamites: or a Demonstration of the 
Existence of Men before Adam; together with a Study of their Con- 
dition, Antiquity, Racial Affinities, and Progressive Dispersion over 
the Earth, 1880. 
Prof. G. Frederick Wright, The Ice Age in North America and its 
Bearings upon the Antiquity of Man, 1889; Man and the Glacial 
Period. 1892. 
Prof. Daniel G. Brinton, Races and Peoples: Lectures on the 
Science of Ethnography, 1890. 
