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ILLINOIS ACADEMY OF SCIENCE 
soon understand the latter as well as the others. The value of 
the sedimentary deposits to the student of ancient climates 
cepends upon the fact that the climatic conditions either directly 
or indirectly control most of the physiographic processes, and 
particularly the highly important process of weathering. Thus, 
ir. very dry regions chemical decay cannot have much effect— 
because moisture is the medium upon which it depends. Such 
sediments as are produced, are, therefore, the products of 
abrasion and isolation rather than of chemical dissolution, 
On the other hand, in a hot moist climate under the dense vege- 
tation which inevitably prevails, chemical decay becomes domi- 
nant and it normally results in ferruginous clays which have 
but little resemblance to the rocks from which they were de- 
rived. It is, of course, evident that climatic conditions also 
influence the immediate deposition of sediments, as well as 
their derivation through the process of weathering. Thus a 
sedimentary layer which is deposited under water and in con- 
tact with much decaying organic matter, will necessarily be 
kept in a reduced or unoxygenated condition, whereas if it 
comes to rest on an exposed dry surface where aerated waters 
circulate down through it, the materials are likely to become 
thoroughly oxidized. 
These are merely illustrations of a series of such influences 
of climate on sediments. Of course, there is still a large 
amount of work to be done by way of putting this branch of the 
science of geology ona firm basis. Weare still far short of the 
point where we shall be able to interpret the climatic condi- 
tions from every kind of sedimentary deposit. 
Let us now apply this principle to the detrital formations of 
Alaska. Neglecting the obvious local variations, the sedi- 
mentary deposits of Alaska possess important characteristics 
in common, but as a group they are contrasted with the sedi- 
ments of, say, Southern California or Brazil. In some of the 
present mountain valleys the glaciers are forming till. In 
cthers the products of frost action and of glacial or fluviatil 
abrasion are being swept down by the streams and deposited 
as gravel, sand, and silt. In all of these deposits the mineral 
particles are almost entirely undecayed, and hence still con- 
tain the original ferromagnesian silicates, feldspars, calcite, 
