IOWA ACADEMY OF SCIENCES. 
27 
SECULAR DECAY OF GRANITIC ROCKS. 
BY CHARLES ROLLIN KEYES. 
The surface of Iowa, and of the neighboring states north and 
east as well, is strewn with boulders of granite, diabase, por- 
phyry, and other igneous rocks in various stages of decomposi- 
tion, and of all sizes, from a few inches to 50 feet or more in 
diameter. These are all more or less rounded or subspherical, 
though often flattened. They have all been transported by ice 
from the north. When traced back to their original ledges the 
latter are usually found to be very smooth and fresh, with prac- 
tically no indications of decay. 
Beyond the glacial boundary southward, and in other parts 
of the world when undisturbed, massive crystalline rocks simi- 
lar to those just mentioned are found disintegrated for many 
feet, frequently as much as 100. It is to some of the phenom- 
ena connected with the decay of granitic rocks beyond the drift 
area that the attention is now called. 
On a former occasion* reference was made to phenomena 
illustrating this subject which were shown with remarkable 
clearness at Woodstock and Sykes ville, on the Potapsco river, 
in Maryland, a few miles west of Baltimore. It was stated that 
in the quarries near the former place jointing was conspicuously 
presented, and that the horizontal divisional planes are partic- 
ularly prominent, at first glance give the impression of true 
stratification. These principal joints extended for considerable 
distances. They were crossed by numerous inclined and verti- 
cal planes of natural cleavage, which were usually much less 
prominent than the major lines just alluded to. 
There is another place, and one very much nearer home, in 
which the same phenomena are shown to even greater advan- 
tage. This is in the Iron Mountain region of southeastern Mis- 
souri. Reference has been made of them identically by Spencer, 
*Proc. Iowa Acad. Sci., Vol. I, pt. iii. pp. 22-24. Des Moines, 1893. 
