ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 27 



shearing joints the apparent elongation is due to the fact that 

 the measurements are diagonally across the blocks, instead of 

 following the bedding. 



From figure 9 it is plain that the average theoretical elongat- 

 ing effect of tension joints is directly as the thickness of the 

 layers or formations through which the joints continuously 

 extend, and indirectly as the radius of curvature. In the field 

 it oftens happens that as a result of the position of a layer or 

 formation upon the convex side of the neutral plane of deform- 

 ation, the different blocks are separated from one another on 

 the concave sides of the curves as well as on the convex sides. 



Fig. 10. — Surticial elongation resulting from shearing joints. 



From figure 10 it is plain that the surficial elongating effect 

 of the shearing joints is great in proportion to the displace- 

 ments along the joints, and to their frequency. The apparent 

 length in any case is the sum of the hypotenuses of the right 

 angle triangles {h-\- h'-\-h' ' -\- h' ' ', etc., Fig. 10), the bases 

 of which are the lengths of the blocks parallel to the bedding, 

 and the perpendiculars of which are the displacements. 



It is not to be concluded from these illustrations that there 

 is no crustal shortening as a result of joint folding. Shortening 

 might occur even if the entire bending were accomplished by 

 tensile joint fracturing. Also in the case of the shearing joint 

 fracturing the rubbing of the blocks against one another might 

 produce shortening. 



However, if an estimate of the original surface of the layers 

 were made, upon the supposition that it was as great as it would 

 appear to be if developed on a plane, this would .result in a 



