March 4, 1921] 



SCIENCE 



197 



seem, rock movement discloses zones of in- 

 herent weakness along which the movement is 

 largely concentrated. 



Causes of Movement. — Eock failure is evi- 

 dence of overpowering stresses, but the causes 

 and directions of these stresses are not so 

 clear. Failure on a mountainous or conti- 

 nental scale points to great earth stresses of 

 the kinds which have been variously ascribed 

 to adjustments under gravity between earth 

 masses of differing densities and topographic 

 relief, to adjustments under gravity of a 

 solid shell to a shrinking centrosphere, a com- 

 ception based on the supposed transfer of heat 

 and magmas from the centrosphere outward, 

 to tidal strains, to changing centrifugal pres- 

 sures caused by changes in rate of the earth's 

 rotation, or to combinations of these causes. 



So clear is the evidence that great earth 

 forces of this kind have been operative that 

 other causes of movement have been perhaps 

 underestimated or ignored in explaining local 

 failure. Such are the pressures and changes 

 of temperature attending the extrusion and 

 intrusion of igneous rocks, in the vicinity of 

 which there is often clear evidence of local 

 failui-e, the recrystallization of rocks during 

 long periods causing local changes of volume, 

 the leaching of substances near the surface 

 causing voids and weakness and consequent 

 slump under gravity, and other volume 

 changes imder weathering. When rocks are 

 in a soft and incoherent condition, they are 

 especially susceptible to local stresses. Mud, 

 marl, sand and salt deposits crumple and slip 

 as the deposits are slowly built up, either 

 under air or water. Local loading by water 

 and ice or rock materials may cause them to 

 fail. Unconsolidated glacial deposits show a 

 variety of joints, faults, and folds. In the 

 settling, consolidation, and desiccation of soft 

 deposits, stresses are set up resulting in local 

 failure. When the deposits are seen later as 

 hard rocks it is difficult to determine the ex- 

 tent to which the failures are to be attributed 

 to these early and local causes acting during 

 the soft formative stages, and to what extent 

 they are the result of regional deformation 

 after the rocks are strong and hard. 



The part played by the forces of crystalli- 

 zation in initiating earth stresses is yet but 

 little understood. Growing crystals have been 

 fomid experimentally to exert considerable 

 linear forces. There seems to be evidence in 

 rocks that these forces have been sufficient to 

 widen openings or to expand the rock mass. 

 Crystallization may also contract the rock 

 mass. The impressive manner in which cry- 

 stal habit asserts and maintains itself under 

 most intense conditions of metamorphism 

 seems to indicate the reaction of considerable 

 forces of crystallization to external environ- 

 ment. It is the custom usually to explain 

 such facts on the basis of adaptation to en- 

 vironment, and to put the emphasis largely on 

 the environmental conditions as determining 

 the outcome. It is clear, however, that these 

 conditions have not been sufficiently intense 

 to interfere with or overcome the tendency of 

 crystals to take whatever form best suits 

 their atomic structure — in other words, to de- 

 velop their o^vn habit. The philosophy of the 

 precise relations between inherent crystallizing 

 power and environmental forces is not under- 

 stood, but enough is known tct warrant the 

 suspicion that the cumulative eilects of the 

 forces of crystallization may themselves 

 initiate earth stresses of a high order of 

 magnitude. 



In my own structural field work, I have 

 become impressed with the necessity for better 

 criteria for the separation of rock structures 

 due to local causes of the kind above indi- 

 cated from the results of failure under the 

 greater regional earth stresses. Of course 

 there is no clean-cut separation between the 

 two. An accumulation of minor and local 

 causes may cause relatively large earth move- 

 ments, and conversely major earth movements 

 are resolved into a complex of minor related 

 structural phenomena. 



Angular Relations of Rock Structures to 

 Causal Stresses. — Just as structures in them- 

 selves do not indicate all the causes of failure, 

 neither do they indicate clearly the directions 

 of application of stress. On the whole the 

 geologist's attempt to relate specific structures 

 with specific stress systems has not been 



