THE MECHANICAL INTERPRETATION OF JOINTS 727 



When we plot the attitude of these joints, using the stereographic 

 projection as explained on page 10 of this paper, we find that the 

 greatest (compressive) principal stress should have been directed 

 upward from the southwest at an angle of 15° if we assume the 

 E-W joints to dip 70° north, and of 30°, if they dip 50° north. 



This correspondence is quite striking and leaves little doubt 

 as to the correctness of this interpretation. 



The northwest-southeast tension, indicated by the position of 

 the obtuse angle of the joints in the Apostle Islands, is parallel 

 to the gentle dip of the rocks of the Bayfield Group. Both dip 

 and tension probably resulted from settling along the axis of the 

 Lake Superior syncline simultaneous with the last movements 

 along the thrust-plane. 



d) The last two examples serve well to show how important it 

 is that each observation of jointing be studied in its geographical 

 and structural relations to all others. To assemble the joints 

 observed over a large area in a single diagram means to veil their 

 true relationships. A diagram, published by Hobbs in 1905,'' 

 shows the strike of 1,004 joints measured by Mr. C. G. Brown 

 in the vicinity of Cajoiga and Seneca lakes. New York, It clearly 

 indicates the existence of two nearly orthogonal double systems 

 of conjugate joints. A comparison with Miss Sheldon's diagram^ 

 shows that the earlier diagram represents a composite picture of 

 the joint systems of two different localities, since the vicinity of 

 Cayuga Lake exhibits only one of the two double systems, as 

 described above. 



A fine example of carefully recorded data giving definite 

 measurements of strike and dip, and accurate geographical location 

 of every joint measured, may be seen, for instance, in R. S. Tarr's 

 field observations embodied in Shaler's "Geology of Cape Ann."^ 



3. Greatest compression horizontal, least compression vertical. — ■ 

 The position of the principal stresses and of the resulting planes 



MV. H. Hobbs, "Examples of Joint-controlled Drainage from Wisconsin and 

 New York," Jour. Geol., Vol. XIII (1905), P- 37o- 



* Loc. cit., p. 66, 



3N. S. Shaler, "The Geology of Cape Ann, Massachusetts," Ninth Ann. Rep., 

 U.S.G.S. (1889), pp, 597-602 and Pis. 72-74- 



