L1RR4 



ON THE FORMATION OF CAVITIES IN ROCKS. 13 



sedimentary rocks, and probably afford many of the regular planes 

 of separation which are often seen crossing one or several beds. 

 They are chiefly due to drying and consolidation. Both the joints 

 formed by cooling and those formed by drying may be afterward 

 modified or increased by rock movements, so that it may be a 

 matter of difficulty to decide between the two forms of origin. 

 The undulatory tremors of an earthquake have been cited, with 

 great reason, by W. O. Crosby as a prolific cause of joints.* 



1.02.02. Cavities Formed by More Extensive Movements in 

 the Earth* s Crust. The strains induced by the compression in the 

 outer portion of the earth are by far the most important causes of 

 fractures. The compression develops a tangential stress which is 

 resisted by the archlike disposition of the crust. (By the term 

 " crust " is simply meant the outer portion of the globe without 

 reference to the character of the interior.) Where there is insuf- 

 ficient support, gravity causes a sagging of the material into syn- 

 clinals, which leave salient anticlinals between them. Where the 

 tangential strain is also greater than the ability of the rocks to 

 resist, they are upset and crumpled into folds from the thrust. 

 Both kind of folds are fruitful causes of fissures, cracks, and 

 general shattering, and every slip from yielding sends its oscilla- 

 tions abroad, which cause breaks along all lines of weakness. The 

 simplest result, either from sagging or from thrust, is a fissure, on 

 one of whose sides the Avail has dropped, or on the other of which 

 it has risen, or both, as will be more fully described under " Faults." 

 If the rocks are firm and quite thickly bedded, as is the case with 

 limestones and quartzites, the separation is cleanly cut ; but if 

 they are softer and more yielding, they are sheared downward on 

 the stationary or lifting side, and upward on the one which rela- 

 tively sinks. Such fissures may pass into folds along their strike, 

 as at Leadville, Colo. 



* In addition to the usual text-books the following references may be 

 consulted : W. O. Crosby, " On the Origin of Jointed Structures," Boston 

 Soc. Nat. Hist., XXII., October, 1882, p. 72; Amer. Jour. Sci., III., xxv. 

 476 ; G. K. Gilbert, " On the Origin of Jointed Structures," Amer. Jour. Sci., 

 III., xxiv. 50, and xxvii. 47; J. H. Kinahan, Valleys, and their Relations 

 to Fissures, Fractures, and Faults, London, Triibner & Co. See also a 

 short letter in the Amer. Jour. Sci., III., xxiv., p. 68, on the " Origin of 

 Jointed Structures;" J. Leconte, "Origin of Jointed Structure in Undis- 

 turbed Clay and Marl Deposits," Amer. Jour. Sci., III., xxiii. 233 ; W. J. 

 McGee, " On Jointed Structure," Amer. Jour. Sci., III., xxv. 152, 476. 



