THE MECHANICAL INTERPRETATION OF JOINTS 723 



The very regular systems of joints observed at station i 

 obviously owe their existence chiefly to the compressive stress 

 caused by the upward buckling of the strata farther north along 

 the fault. The change of the angle of shearing from 65° at station 

 I, to 90° at stations 2 and 3, probably is due to factors which will 

 be discussed in the second part of this paper. It is brought about 

 by a shifting of the trend of the system s, which at the same time 

 becomes more and more irregular and scattered. At station 4 

 set S2 is only represented by a few widely separated cracks, while 

 a new system of fractures makes its appearance. They are parallel 

 to the fault and become increasingly prominent and closely spaced 

 as the fault is approached. They must be tension cracks. 



The joints at station 5 ofl"er an exact analogy to the fan-shaped 

 cracks formed on the upper surface of a glass plate under torsion, 

 with shearing planes developed only on one side of a tension crack. 



The great practical value of such an interpretation of joints, 

 during the progress of field work, is obvious. In this case, for 

 instance, the joints at stations i to 3 would lead the field geologist 

 at once to look for an uplift either north or south of these points, 

 or both. The sudden appearance and increasing importance of 

 an additional system such as the joints of set t would suggest the 

 neighborhood of a flexure or a fault not far to the northwest striking 

 N65E. 



Information such as this will certainly pay for the time spent 

 on detailed intelligent observation. 



b) The remarkable jointing exposed along the shores of Lake 

 Cayuga and Lake Seneca, "resembling the gigantic ruins of Cyclo- 

 pean architecture," has been made classic through the series of 

 woodcuts published by Hall in 1843.' 



Miss Pearl Sheldon^ has given us a large number of careful 

 measurements of these joints in the vicinity of Cayuga Lake. 

 Two systems of joints stand out from all others. They are gen- 

 erally stronger, more regular, and remarkably constant in their 



' Geology of New York, Pt. LV (Albany, 1843), PP- 303-6. For good modern 

 illustrations see, e.g., Watkins Glen-Catatonk Folio, No. 169 (1909), PL I, Figs. 15 and 

 16; and Jour. Geol., Vol. XX (191 2), p. 78. 



^ Pearl Sheldon, "Some Observations and Experiments on Joint Planes," Jour. 

 Geol., Vol. XX (1912), pp. 53-79; 164-83. 



