IGO JVisconmn Acade^ny of Sciences, Arts, and Letters. 



tion has been given tliat complex tlirnsts acting on strata are de- 

 composed into two directions at right angles to each other. One 

 of these is the direction of maximum thrust, and the other is the 

 direction of minimum thrust. At right angles to the maximum 

 thrust the more numerous and larger folds are formed, and at 

 right angles to the minimum thrust lesser folds are formed. ^ 



Another very interesting phenomenon is the combination of 

 folds and faults. When the ice or shore is first deformed, or is 

 deformed slowly, it often yields by bending or by flowage. 

 When, however, bending goes too far or the process of deforma- 

 tion becomes too rapid, faulting takes place. Faults form par- 

 allel to the axis of the folds, giving overthrust faults on the anti- 

 clines ; they also form at right angles to the folds, giving cross 

 faults. These phenomena precisely duplicate similar phenom- 

 ena shown by many mountain masses. As Dr. Buckley says, 

 the deformation of ice and shore illustrates very well, indeed, the 

 phenomena exhibited by rock deformation in the zone of com- 

 bined fracture and flowage. 



In this connection a point of very great interest is the sud- 

 denness wdth which the rujotures frequently take place in the de- 

 form^ation of the ice. Dr. Buckley's explanation of this is that 

 the tensile and compressive stresses accumulate until the ultimate 

 strength or elastic limit of the ice is reached, when release must 

 come, either by tensile or compressive ruptures. Precisely the 

 same state of affairs takes place in rocks. They have consider- 

 able elasticity, and as the earth's stresses gradually increase the 

 molecules are pulled apart or brought closer together, until fin- 

 ally the elastic limit is reached, when tensile or compressive rup- 

 ture occurs. The evidence that the crust of the earth is under 

 compressive stresses within the elastic limit is conclusively 

 shown by the fact that ruptures are sometimes known to take 

 place by the layers of rock rising and breaking when there is re- 

 lease of load by excavation. As, for instance, in the Chicago 

 drainage canal, as noted by Salisbury, at the combined locks of 

 Appleton, as noted by Cramer, ^ and elsewhere. 



Another of the interesting phenomena observed and described 



1 Principles cit., pp. 626-627. 



^Ona Recent Rock Flexure; by Frank Cramer, Am. Jour. Sci. iii, Vol. XXXIX, 1890» 

 pp. 220-225. 



