7o8 



WALTER H. BUCHER 



It was found that (a) The jointing is confined to the upper 

 third (or even less) of the massive sandstone which is here about 

 loo feet thick. It is entirely lacking below. (&) It marks the 

 crest of a minor anticline on the downthrow side of a conspicuous 

 fault, (c) The average hade of the joint-traces on the practically 

 vertical exposure which trends about in a NNE-SSW direction, is : 

 set I: 27°— NNE; set II: 35°— SSW; inclosed angle: 62°. (d) The 

 average strike of the joint- traces, measured on the horizontal surface 

 of projecting ledges, is: set I: N 78 W; set II: N 27 W; inclosed 

 angle: 51°. (e) The joints of set I are much better developed, 



Fig. I. — Jointing in vertical cliff of massive, cross-bedded sandstone on Mine 

 Fork, Magoffin County, Kentucky. 



longer, more continuous and more regular in their course than 

 those of set II, both in the vertical and in the horizontal planes. 



For two reasons the occurrence of this system of joints at this 

 locality seemed surprising. There could be little doubt that 

 these joints represented planes of shearing. The writer had, 

 however, always associated the fracturing of hard materials by 

 shearing with compressive stresses or, at best, with compound 

 stresses resulting in torsion. But here he was dealing with a 

 clear case of simple tension along the crest of an anticline, causing 

 a hard sandstone to fail along typical planes of shearing. 



He had also been accustomed to ascribe to the planes of maxi- 

 mum shear a general tendency to intersect at right angles. No 



