Hobbs — The Correlation of Fracture Systems . 21 
probably if the joints could be measured with greater accuracy 
they would fall between two five degree limits on the side of the 
one having the numerical superiority. 
Iowa. —McGee 1 has called attention to the conjugate set of 
joints which penetrate the quarry rocks of the State of Iowa. 
Of these joints there are two classes recognized, which are desig¬ 
nated by the quarrymen as “clay seams” and “dry seams.” 
The chief set consists of two series of “clay seams” crossing 
each other approximately at right angles, with two series of “dry 
seams” also crossing about at right angles and approximately 
bisecting the angles formed by the principal series. He further 
adds: 
“The phases presented are, however, variable. Either of the 
two classes or either system of either class may be absent, or 
additional and generally iess conspicuous systems . . . some¬ 
times so blended with the predominant class as to separate the 
strata into either irregular or tolerably regular polygonal blocks 
. . . may be introduced; and the two classes pass impercep¬ 
tibly into each other.” 
In a personal letter Dr. McGee adds: 
“Unhappily my memory is not as clear as I could wish; so far 
as I am able to recall the less distinct double-sets are orthogonal, 
departing a few degrees from N-S and E-W, though the direc¬ 
tion of the departure from the meridian and parallel escapes me. 
The more prevalent and conspicuous doubie-sets are also orthog¬ 
onal or nearly so and approximately NW-SE, and NE-SW. So 
nearly as I can recollect the minor and major double-sets do not 
exactly divide the quadrangle. The difference is somewhat more 
(or less) than 45°; . . . 
“As I dwell on the matter in this writing the impression re¬ 
vives that the dominant single-set trends about N. 55° W., and 
S. 55° E., with the correlative orthogonal single-set about N. 35° 
E., and S. 35° W.; and that the stronger joints of the minor 
double-set trends about N. 5° W. to S. 5° E., and the weaker one 
about N. 85° E. to S. 85° W. In any event such a scheme will 
i McGee, W. J., Note on jointed structure. Am. Jour. Sci. 3rd Ser. 
Vol. 25, 1883, pp. 152-155. 
