THE MECHANICAL INTERPRETATION OF JOINTS 725 



direction, the contact rises rapidly to over 2,100 feet in a similar 

 distance. A closer inspection of the northeast corner of Catatonk 

 Quadrangle reveals the presence of considerable doming in the 

 general direction suggested by the position of the joint planes. 

 This broad anticlinal bulge does not seem to be mentioned in. the 

 text of the folio. The fact that the writer's attention was called 

 to it through the analysis of the joints of Cayuga Lake, serves 

 well to illustrate the practical possibilities of the method employed. 



The presence of an uplift to the northeast accounts for the 

 existence of a compressive stress in that direction. The horizontal 

 tensile stress implied by the position of the joint planes can be 

 accounted for equally well. Crossing the shores of Cayuga Lake 

 in a southeasterly direction (suggested by the obtuse angle of 

 the joints), we find that, on the crests of the Fir Tree Point, 

 Watkins. and Alpine anticlines, the contact of the Portage and 

 Chemung formations remains essentially at an elevation between 

 1,600 and 1,700 feet above sea;level. Beyond the Alpine anti- 

 cline, however, in the same southeasterly direction, within a 

 similar distance, the same contact drops to near 1,000 feet in the 

 vicinity of Jenksville in Newark Valley Township. 



The existence of this depression in. the direction suggested by 

 the position of the joint planes, leaves little doubt that this 

 relatively pronounced flexure gave rise to the tensile stress involved 

 in the formation of the joints. 



c) For a last example we turn to Thwaites's paper on the 

 "Sandstones of the Wisconsin Coast of Lake Superior."' 



When we plot the strike of the joints of this region as recorded 

 in the table on page 96, it appears that the peninsula north of 

 Washburn, including the Apostle Islands, in contrast to the regions 

 to the west and south, is traversed by two dominant and persistent 

 systems of major joints. One of the two strikes on the average 

 E-W, the other about 10° east of north. Most probably they 

 represent planes of shearing. The position of the acute angle 

 points to the action of a compressive stress in a NE-SW direction, 

 with a tensile stress acting in a NW-SE direction. 



' F. T. Thwaites, "Sandstones of the Wisconsin Coast of Lake Superior," Wis. 

 Geol. and Nat. Hist. Sur., Bull, 25 (191 2). 



