196 



SCIENCE 



[N. S. Vol. LIII. No. 1366 



whole. The necessity of integrating evidence 

 and information from scant observations re- 

 quires an understanding of the interrelations 

 of structures and of great group characteris- 

 tics of a given environment or of a given kind 

 of rock. I would like to comment briefly on 

 some of these broader considerations, not ex- 

 haustively, and certainly not with full under- 

 standing, but with a view to indicating some 

 of the salient facts now known and the man- 

 ner in which these facts have been built into 

 certain generalizations and hypotheses as to 

 movements of the lithosphere. 



I. STRUCTURAL FAILURE IN THE ZONE OF 

 OBSERVATION 



We may direct our attention first to the 

 structural failure of rocks extending' down- 

 ward only a few miles from the earth's 

 surface. The characteristics of this r^ion 

 are disclosed to us by deformed rocks, some 

 of which were once far below the surface, but 

 now brought within our range of observation 

 by the erosion of overlying rocks. This may 

 be conveniently referred to as our zone of ob- 

 servation. 



Heterogeneous Nature of Movement. — In 

 this zone, some of the rocks have been de- 

 formed by rock flowage and some by rock 

 fracture, both kinds of deformation often re- 

 sulting in folding and tilting of beds. By 

 rock flowage we mean " solid," " plastic," 

 " massive," or " viscous " movement under 

 great containing pressures during which the 

 rock and its constituent minerals retain their 

 properties of elasticity and rigidity. 'No one 

 of these descriptive terms may be technically 

 accurate and comprehensive, but the move- 

 m.ent partakes of the characters expressed by 

 all of them. The movement is not necessarily 

 slow and continuous; there is geologic evi- 

 dence that it is periodic. Rock flowage is 

 essentially characterized by the parallel di- 

 mensional arrangement of minerals, like 

 mica and hornblende, developed by recrystal- 

 lization during the process. These minerals 

 are present abtmdantly after the process, not 

 before. Eock flowage is intimately associated 

 with fracture, including the minute granula- 



tion and slicing of mineral particles, and in- 

 cluding larger fractures, especially of the 

 shearing type. While rock flowage and rock 

 fractiu-e constitute two distinct types of de- 

 formation, there is almost complete gradation 

 between the two, and much deformation is 

 not accurately described by either term. A 

 displacement may take place along a clean 

 fracture, or along a fracture on which there 

 has been local rock flowage, or along a zone 

 of closely spaced parallel fractures with rock 

 flowage affecting all of the intervening 

 masses, or along a zone of rock flowage in 

 which evidences of fracture planes are in- 

 distinct or altogether lacking. A single shear 

 plane may show all of these features. In a 

 large way a considerable zone of flowage may 

 often be interpreted, in its relations to dis- 

 placement and stresses, in much the same 

 manner as a fracture plane. 



The difiiculty of a precise definition of the 

 two phenomena of fracture and flow is well 

 illustrated in the so-called flow accomplished 

 experimentally. Shearing, thrust, granulation 

 and slicing are here strongly in evidence, 

 while the parallelism of mineral particles 

 brought about throug'h reerystallization, so 

 conspicuous in schists, slates and gneisses, 

 which are the principal evidence of rock flow- 

 age in nature, is almost lacking in the experi- 

 mental results. Deformation induced artifi- 

 cially is plastic flow, but the same kind of 

 deformation observed in nature is often called 

 fracture. With a longer time factor the ex- 

 perimental flowage would presumably more 

 closely approximate that of nature. 



Structural failure within our zone of obser- 

 vation, whether by fracture or flow, has not 

 been confined to any particular plane or 

 formation, but is so distributed as to indi- 

 cate that adjustment of rock masses under 

 deforming stresses has been accomplished by 

 movement in many zones, in many forma- 

 tions, in all directions, and with all inclina- 

 tions. Rocks in this zone as a whole have 

 not yielded to stresses as homogeneous masses. 

 In fact, even down to comparatively small 

 units of volume the rule is heterogeneity. No 

 matter how homogeneous the formation may 



