Makch 4, 1921] 



SCIENCE 



207 



the zones of movement. This conception is 

 taken to afford the best initial basis for the 

 interpretation and correlation of observed 

 rock structures. There is no certain evidence 

 of increase or decrease of movement toward 

 the bottom of this zone. Beyond a shallow 

 surface zone, there is no certain evidence of 

 increase of rock flowage and decrease of rock 

 fracture with depth. There is no certain evi- 

 dence that rock flowage means greater weak- 

 ness than rock fracture There is no certain 

 evidence in rock flowage that pressures are 

 dominantly hydrostatic or dominantly those 

 of competent solid bodies. 



Movements are known to occur in the zone 

 below our range of observation, but their 

 nature and distribution are the subjects of 

 varied hypotheses based on a few known con- 

 ditions. Much of the sharper diastrophism 

 seems to be confined to a thin surficial zone. 

 Deeper movements, of a more massive type, 

 periodic, and possibly slower, seem to be im- 

 plied by the relative movement of great earth 

 segments as represented by continents and 

 ocean basins. Their depth is unknown. Most 

 of the current hypotheses agree in assuming 

 a single mobile zone in which rocks move 

 dominantly by rock flowage. The basic re- 

 quirements of reasonable hypothesis, however, 

 may be equally well met by a conception of 

 movement much like that of the zone of ob- 

 servation. This does not require or postulate 

 the conception of the existence of any single 

 mobile zone, or zone of slipping, or zone of 

 flowage, or of an asthenosphere. It supposes 

 movement irregularly distributed in many 

 zones, with any inclination, and accom- 

 plished by both fracture and flowage as far 

 below the surface as movement extends — 

 always remembering that some of the struc- 

 tures geologically described as fractures, may 

 be expressions of mass movement of the kind 

 defined as flow in exx)erimental results. 



Conditions of temperature and pressure 

 and vulcanism become more intense with 

 depth, but it remains to be shown that their 

 conjoint action results in a uniform environ- 



ment, and even if it does, that this condition 

 is not upset by what might be called a hetero- 

 geneity of the time factor as represented by 

 differing rates of deformation. If homogene- 

 ous environmental and time conditions are 

 assumed, it is yet to be shown that these are 

 sufficient to overcome the heterogeneity of the 

 physical properties of the rocks and to cause 

 homogeneous behavior through any consider- 

 able zone. It is not even certain that they 

 may not fix and accentuate the heterogeneous 

 properties of rocks. Certainly in the zone of 

 observation there is comparatively slight evi- 

 dence of their efficacy in causing more uni- 

 form deformation with depth. 



In short, as between alternative conceptions 

 as to the conditions in the deep zone, the bur- 

 den of producing affirmative evidence would 

 seem to rest heavily on any conception involv- 

 ing radical departure from the known irregu- 

 lar distribution and manner of movement 

 within our zone of observation. We come, 

 therefore, to the Chamberlin conception of a 

 heterogeneous structural behavior of the earth. 

 C. K. Leith 



Universitt op Wisconsin 



SCIENTIFIC EVENTS 



DINNER IN HONOR OF THE RETIRING 

 SECRETARY OF AGRICULTURE 



The success of Secretary E. T. Meredith in 

 interesting the public in the investigational 

 work of the U. S. Department of Agriculture 

 has been unique. His prompt recognition of 

 the needs of the department and his activity 

 in behalf of the investigators there, have 

 attracted the attention of scientific men 

 throughout the country. Coming to the secre- 

 taryship at a time when the morale of the 

 scientists in many government departments 

 was being seriously impaired through dis- 

 couragement as to the possibility of securing 

 adequate support for investigation, his cam- 

 paign of education had the effect both of 

 awakening the public to the extent and im- 

 portance of the work, and of heartening the 

 workers. 



