24 C. R. VAN HISE 



gentle practicable curves should be assumed to the places where 

 the formation reappears, and even if this be done, as shown (pp. 

 20-22) an overestimate may be made of the length of the for- 

 mation. 



5. It should be ascertained whether the formation measured 

 has upon the average been thinned or thickened, and a corre- 

 sponding allowance should be made. 



If the principles are not appreciated upon which the fore- 

 going precautions are based, with the natural, indeed almost 

 inevitable tendency for one to pick out strata for measurement 

 which have suffered severest deformation here, and severest 

 deformation there, we may be sure that estimates of shortening 

 will have comparative little value. 



Jointing. — In another place 1 I have explained that joints may 

 be of two kinds, tension joints and compression joints. Tension 

 joints in simple folds may form in one direction at right angles 

 to the bedding, or nearly so, in the zone of fracture (Fig. 8). In 

 the case of complex folding, two sets of tensile joints intersect- 

 ing each other at right angles may develop, both, however, still 

 normal to the bedding or nearly so. Compression joints, form- 

 ing in shearing planes, are ordinarily more or less diagonal to 

 bedding. However, the greatest compressive stresses may 

 approximate angles of 45 ° to the bedding, in which case the 

 shearing fractures would be nearly normal to bedding. Com- 

 pression joints, like tension joints, may develop in two direc- 

 tions at right angles to each other. 



In the gentle folds of the Paleozoic of the Mississippi valley 

 and the strata of the plateau country of the far West, joints are 

 normal to the bedding, or nearly so, corresponding in position to 

 the direction of the folding. For instance, southern Wisconsin 

 is a gentle southward-plunging anticline, in other words, the prin- 

 cipal fold has a nearly north-south axis, and the rocks dip east to 

 Lake Michigan and west to the Mississippi river. Corresponding 

 to this arrangement are numerous joints in a north-south direc- 



1 Principles of North American pre-Cambrian geology, by C. R. Van Hise : 

 1 6th Ann. Rept. U. S. Geol. Survey, Pt. I, 1896, pp. 668-672. 



