R. Pumpelly—Secular Rock-disintegration. 141 
material operated on. It is evident, I think, that we have in 
these residua an adequate source of material for the glacial drift 
and for the loess, although other sources have probably con- 
tributed in some regions to the formation of the latter, as for 
instance, rivers which, fed by silt-loaded glacier-streams,. sink 
away on the plains of the central area, leaving their suspended 
material to dry and drift on the surface. 
n reply to some questions bearing on the possible sources of 
the loess of the Missouri valley, Mr. Clarence King kindly 
informs me that during the Pliocene the entire surface of the 
great plains was covered by a sheet of water which he has 
called Cheyenne Lake. Its deposits, known as the Missouri 
Pliocene—2,000 feet thick at the western edge along the Rocky 
Mountains—become very thin at the eastern margin in Kansas 
and Nebraska. Throughout the eastern portion at all points 
over 150 miles east of the mountains, the material is exceed- 
ingly fine—so fine that Professor Brewer found that when 
shaken with water in a test tube it does not settle after stand- 
ing a year. Furthermore, Mr. King (anticipating one of the 
plains. During this period there were expose to -erosion, 
not only the surface of the plains, but the entire region east 
and west of the mountains that had been subjected e Bad- 
land erosion. The countless cliffs and rounded outlines sculp- 
tured out of the soft Tertiary clays and marls which form the 
ad-lands were particularly susceptibleto wind-erosion. Finally, 
Mr. King summarizes his answer in these words: “The lacus- 
trine basins of the plains and Rocky Mountains certainly afford 
sufficient fine material, the period of desiccation between the 
two floods of the Quaternary affords the necessa 8S, a 
the prevalent west wind would account for the transportation 
to the east.” 
evidence of this power. It seems to me rather that the abun- 
ance of the drift material may be equally well taken as in 
great part but not wholly a measure of the effect of long-con- 
tinued and undisturbed disintegration and of the transporting 
Power of ice, and that a period of glaciation may be, as Ruti- 
meyer insists, a period of comparative rest as regards excava- 
tion of hard rock. ; / 
The great source from which mountain glaciers derive their 
Moraines is, as is well known, in the incessant rending action 
of cold—of constantly alternating contraction and expansion— 
Am. Jour. —,, — Vou. XVII, No. 98.—Fes., 1879. 
