172 



WILLIAM H. HOBBS 



or thrust, and that in successive stages of its evolution the anticline 

 became increasingly unsymmetrical with the axial plane dipping 

 away from the active force and finally was underthrust in the same 

 sense. In recent experiments by Paulcke' which were carried out 

 with the idea of simulating Alpine tectonics, hut with the presupposi- 

 tion that there had been overthrusting from behind the anticlines, the 

 arches were in normal cases generally either bent over toward the 

 active (moving) force, or else a stiff plate {Druckplatte) was intro- 

 duced and prevented their natural manner of deformation. 



From the nature of rock materials we conceive that folds develop 

 within a zone probably some miles below the earth's surface, since 

 we beUeve that at such depths only can the rocks become sufficiently 

 plastic under their load. Nearer the surface rock materials, which 

 are normally highly elastic, must be deformed by failure or fracture, 

 and over a rising anticline must be adjusted in block sections whose 

 movements become manifest at the surface in earth shocks or 

 quakes (note conditions in Bonin arc, p. 79). 



A consequence of the studies by Adams, which have for the 

 first time revealed the enormous hydrostatic compressive strengths 

 of rocks, is certain to be a modified conception of the zone of flow 

 (better, zone of folding) within the earth's surface shell. To as- 

 sume that more than eleven miles of sediments have been eroded 

 from those folded beds which outcrop at the earth's surface must 

 make the hardiest theorist pause and consider whether the closing 

 of pores necessary to permit of folding may not be due to an excess 

 of tangential compressive force over that of the radially directed 

 load— or, in other words, that hydrostatic conditions of compres- 

 sion were seldom reahzed in the folding of those strata, at least, 

 which we find exposed at the earth's surface. 



THE FOLDING PROCESS STUDIED IN THE PROFILE — ANTICLINE 



EVOLUTION 



Successive sectional curves of a growing anticline. — Since, when the 

 antichne begins to rise, the radial component of the compressive 

 stress has the least value, and the tangential component the 



' W. Paulcke, "Das Experiment in der Geologic," Festschrift z. Feier des Geburts- 

 tags Seine Koit. Hoheit, etc., herausg. v. d. Tech. Hochschule, Karlsruhe, 191 2, pp. 108, 

 figs. 44 and pis. 29. Note the Druckplatte in the apparatus which is figured in PI. 8. 



