860 STUDIES FOR STUDENTS 



planet. Strains brought about by this process indicate an inclination to dif- 

 ferentiate themselves into tangential and radial strains, thereby inducing 

 horizontal (that is, compressive and folding), and vertical (that is, depres- 

 sional) movements. Displacements have therefore been separated into two 

 principal groups, by one of which have been produced more or less hori- 

 zontal, by the other more or less vertical changes of position of portions of 

 the crust. 



Under compressive stresses rocks may suffer deformation in 

 one or both of two ways : by flow, when under sufficient load ; 

 and by rupture, when the load is insufficient to cause flow. 



DEFORMATION ZONES OF THE LITHOSPHERE. 



It has been shown that at a depth of 1 0,000 meters, more or 

 less, even the sirongest rocks must find relief from stress by 

 flow, and hence below that depth there must be a zone which, 

 as respects its manner of deformation, may be called the zone of 

 flow. 1 At very moderate depths relief from stress will be 

 obtained by rupture, and this uppermost belt is therefore denom- 

 inated the zone of fracture. Since, however, the depth at which 

 rocks will flow is dependent primarily upon their crushing 

 strength, there will be an intermediate zone or belt within which 

 deformation will take place partly by rupture and partly by flow 

 (zone of combined fracture and flow). There are many othei 

 conditions — such as amount of contained water, elevation of 

 isogeotherms, etc. — which modify the depths of the zone of flow, 

 but they need not be taken up here. 



It is clear from these considerations that rocks may be 

 deformed at one period by fracture (when under light loads), 

 and at another time by flow (when below a depth of 10,000 

 meters, more or less). During a general elevation of a region 

 deformation by flow within a given rock mass, will in general 

 precede that by fracture, and the structures which arise from 

 fracture will be superimposed upon those developed by flow. 

 During a considerable subsidence of a province structures due 

 to flow will be superimposed upon fracture structures and either 

 wholly or partially efface them. The presence in the same 



*Van Hise, " Principles of North American Pre-Cambrian Geology," Sixteenth 

 Ann. Rept U. S. Geol. Survey, Part I (1896), pp. 5^9-94- 



