Studies for Students. 



DEFORMATION OF ROCKS. V. 



SUPPLEMENTARY NOTES.' 



Separation of the outer part of the earth into zones. — In dividing 

 the outer part of the crust of the earth into an upper zone of 

 fracture, a middle zone of combined fracture and plasticity, and 

 a lower zone of plasticity,^ three factors should be taken into 

 account, ( i ) the depth of burying, and therefore the vertical 

 jpressure, (2) the relative strength and plasticity of the materials, 

 ■and (3) rapidity of deformation. 



If the last two factors were constant, as a result of the 

 ■first factor the zone of plasticity would be directly below the 

 zone of fracture with a possible narrow transition zone. The 

 greater the strength of materials, and the greater the rapidity of 

 (deformation, the deeper is the zone of plasticity. The weaker and 

 imore plastic the materials, and the slower the deformation, the 

 nearer the surface is the zone of plasticity. However, as these 

 factors vary greatly there is a wide middle zone of combined 

 fracture and plasticity. Some rocks may be deformed by plastic 

 flow very near the surface, and others by microscopical fractur- 

 ing at a great depth. As illustrating this, a bed of mud may be 

 deformed without fracture at or near the surface. Upon the other 



^ Figure loi, on page 595, of my paper on the Principles of North American pre- 

 Cambrian Geology, in the Sixteenth Ann. Rept. of the U. S. Geol. Survey, Part I, was 

 taken from Dr. Carl Futterer. The statement, "After Futterer" was in the manu- 

 script list of illustrations, but by mistake this was omitted in printing. 



^ (A) Principles of North American pre-Cambrian Geology, by C. R. Van Hise ; 

 with an appendix on Flow and Fracture of Rocks as related to Structure, by L. M. 

 HosKiNS. Sixteenth Ann. Rep. U. S. Geol. Surv., Part I, 1896, pp. 589-603. 



(B) Deformation of Rocks, by C. R. Van Hise. Jour, of Geol., Vol. IV, 1896, 



pp. 195-213- 



178 



