320 CALIFORNIA ACADEMY OF SCIENCES. 



minute slippings, the layers in time become folded, and 

 when the movement has ceased and erosion exposed 

 them, they appear intact. 



In many cases, as shown b}' microscopic examination 

 of the rocks, this may be the true explanation,^ but of 

 late the belief has been gaining ground that by introduc- 

 ing the factor of viscosity of solids under stress the same 

 results may be obtained without resorting to fracturing 

 at all. ^ 



It has long been known that in igneous and meta- 

 morphic rocks when heated, the particles have a certain 

 freedom of motion among themselves called flow, similar 

 to the flow of any plastic substance like putty under the 

 slight pressure of the hand. * 



M. Tresca and others have shown that similar flow 

 occurs in cold solid bodies when subjected to a pressure 

 above the elastic limit and below the ultimate strength.^ 



In application of this principle, if we suppose a hori- 

 zontal pressure acting upon a layer of rock which at 

 some point is not quite horizontal, the initial pressure 

 will at that point be resolved into two components, one 



6 Geikie's Textbook of Geol., pp. 29-2-3, 506, 575 aud 578. 



" Eeade's Origin of Mouutaiu Eauges, pp. 91 aud 92; Becker in Bull. 

 Geol. Soc. Am., Vol. 4, 1893, pp. 13-90. 



» King, U. S. Geol. Exp. 40th Par., Vol. 1, p. 752; Callaway in Quat. 

 Jour. Geol. Soc, 1883, p. 383; Barus in Am. Jour. Sci., Ill, Vol. 36, p. 

 178; III Vol. 39, p. 234; Prestwick's Geology, Vol. I, p. 304; Heim's 

 Mechanics mus der Gebirgsbildung, 1878. 



» Tresca in Comtes Kendus, Vol. 59, 1864, p. 754; Vol. 64, 1867, p. 809; 

 Tresca in Mem. Sav. Etraugers XVIII, p. 733; XX, p. 75; Tresca in lust. 

 Mech. Eng., June, 1867; June 1878; Tresca in The Engineer, June 28, 1878, 

 p. 463; June 14, 1878, p. 428; Townsend in Jour. Franklin Inst., March, 

 1878; Williams in Gentleman's Mag., Feb., 188.3, pp. 231-2; Spring in The 

 Engineer, Apr. 9, 1886, p. 278; Barus, Bull. U. S. Geol. Surv., No. 73; 

 Becker in Bull. Geol. Soc. Am., Vol. 4, 1893, p. 51, also p. 13 et seq; 

 Barus in Am. Jour. Sci., Ill, Vol. 45, Feb., 1893, p. 87. 



