30 
IOWA ACADEMY OP SCIENCES. 
number of observations taken in different places, and by differ- 
ent persons, show, according to Haworth, that they have very 
slight variations in direction, the planes trending north 60 
degrees east. Only in a few cases were the deviations very 
marked. The various quarries, of course, exhibit the jointing 
phenomena best, though it is also well shown along the bluffs 
bordering the streams. 
A third series of prominent vertical seams have been made 
out by Dr. E. Haworth, who has been engaged upon a petro- 
graphical study of the rocks of this region, which makes angles 
of about 80 and 100 degrees with those just mentioned. In 
some cases joint planes, very much less pronounced than any 
of the others, have been observed cutting the rock masses into 
rhomboidal blocks, though these are only locally developed. 
With the rock mass subdivided into more or less rectangular 
blocks of varying size, access of metoric waters is readily per- 
mitted and the seams become wider as the blocks begin to break 
down. The edges being attacked from two directions succumb 
more rapidly than the sides, and the corners effected on three 
sides break down still more rapidly. As a result there is even- 
tually formed a more or less spherical mass from each block. 
These rounded masses or boulders are commonly separated 
from one another by greater or less thicknesses of granitic 
sand. (See figure 1, plate hi.) 
If undisturbed the process may go on for a depth of many 
feet, even a hundred or more, as is shown in many localities. A 
vertical section through a place thus decaying, as is sometimes 
shown in a quarry, imparts to the ledge an appearance of a 
great wall of cyclopean masonry. Layer after layer of huge, 
rounded blocks, rising one above another, with the regularity 
and precision of human efforts. The interior of the boulders 
in each case may be perfectly fresh and well adapted for build- 
ing purposes. As the process of decomposition progresses the 
amount of interstitial sand gradually increases and the rounded 
blocks become proportionately smaller. If at these stages run- 
ning water passes over the region, the blocks with rounded 
edges, stand out prominently as the sands around them are 
washed away, giving a striking tesselated appearance to the 
surface, or a characteristic roches moutonees appearance is 
imparted. (Figure 2, plate iii.) If the sand is washed away at a 
later stage the boulders of all sizes are uncovered completely and 
stand out on the less decomposed ledges beneath. Some of 
