28 C. R. VAN HISE 



considerable overestimate of the amount of its original surface. 



In many of the weaker layers of folded rocks is a diagonal 

 jointing, due to differential motion, and cutting this diagonal 

 jointing nearly at right angles is a diagonal fissility. 1 The whole 

 may result in thinning the limbs of the folds, just as does the 

 shearing motion in the case of folding by plastic flow. 



It is very desirable that the quantitative value of the length- 

 ening effect of jointing should be known for various kinds of 

 deformation. This, however, is an exceedingly difficult task. 

 The quantitative value of the surficial elongation due to jointing 

 for any deformation of an area can be only approximately 

 determined after an extensive and close field study of the dis- 

 trict. Consequently, for the present, I am obliged to be content 

 with comparative statements which rest upon my own judgment, 

 and which may be questioned by other observers. I believe the 

 elongating effect of jointing to be quantitatively of sufficient 

 importance that it should be taken into account in estimates of 

 crustal shortening. I believe the lengthening effects of joints 

 are important in connection with the estimates of shortening due 

 to folding where the folds on the flanks of mountains may be due 

 to a downward gliding effect, and be compensated by the joints 

 (see Fig. 7), as explained on pages 20-22. However, I suppose 

 that the elongating effect of jointing is not so great as that of the 

 distortion of closely folded rocks in the zone of flow, as explained 

 on page 16. 



Faults.- — Faults are ordinarily classified into normal faults 

 and reverse faults. The normal faults involve an elongation of 

 the crust of the earth as certainly as the reverse faults involve 

 a shortening of the crust of the earth. 3 The very names, normal 

 and reverse faults, show that the first are of far greater abun- 

 dance, — are in fact the rule. 



•Principles of North American pre-Cambrian geology, by C. R. Van Hise : 16th 

 Ann. Rept. (J. S. Geol. Survey, Pt. I, 1896, pp. 651-654. 



2 Supplementary notes on deformation of rocks, by C. R. Van Hise : Journ. 

 Geol., Vol. V, 1897, pp. 190-191. 



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

 1 6th Ann. Rept. U. S. Geol. Survey, Pt. I, 1896, p. 674. 



