25 
THE RELATION OF SECULAR DECAY OF ROCKS 
TO THE FORMATION OF SEDIMENTS.* 
Kai.ph S. Tark, Ithaca, N. Y. 
CONTENTS. 
I. Secular Disintegration. (b) Derived by S urface Wash. 
(a) Conditions of Decay. (c) Derived by Wind Action. 
(b) Characti'i-istics of Residual Soils. (d) Derived by Ice Action. 
(c) Distribution of Residual Soils. (e) Derived by River Action. 
II. Formation of Sediments. (f) Derived by the Sea direct, 
(a) General Statement. (g) Summary. 
/. Secular Disintegration. 
(a) Conditions of Decay. 
Rocks are composed of minerals in greater or less variet}' both 
as to character and quantity, and the decay of rocks depends upon 
both the chemical and mechanical weakness of the material. 
The chief factors in rock disintegration are the mechanical ef- 
fects of heat, frost and organic life, and the chemical decay of 
minerals. 
Quartzose rocks decay with extreme slowness and their disin- 
tegration is chiefly mechanical. Limestone disappears rapidly, 
but soil accumulates slowly because of the solubility of the com- 
ponent minerals, the insoluble impurities alone remaining. Slates 
and other clay rocks, being themselves largely composed of resid- 
ual products, and hence not subject to extreme chemical altera- 
tion, produce soil slowly except by mechanical means. The rocks 
best fitted for rapid and extensive production of residual soils 
are the crystalline rocks both of igneous and metamorphic origin. 
This is particularly the case with those rich in the lime soda 
feldspar, hornblende and the like. These are broken up by decay 
into the less soluble silicates and the soluble protoxide bases. 
Since a given quantity of a very easih' destroyed rock, such as lime- 
stone, disappears quickly, Init leaves a small residuum, whereas the 
same quantity of a less soluble rock disappears more slowlj- but 
with greater residual product, the resulting amounts of residuum 
in the two cases, in a given time ma}' not be wideh' different. 
The chemical decay of rocks goes on often to a very great 
*This essay was prepared lor another purpose, and is in the main an 
abstract of the present knowledge on the subject. The scattered state 
of the literature on Secular Disintegration, and the general interest of 
the subject, has prompted me to publish this summary where it will be 
accessible. 
