Maech 4, 1921] 



SCIENCE 



205 



speoimen behaving similarly. In fact marble 

 actually penetrated tbe harder diabase. Like- 

 wise, gypsum penetrated steel. While there 

 are probably differences in the internal fric- 

 tion or viscosity of different rocks under these 

 conditions, the results are nevertheless homo- 

 geneous in approximating rock flovcage — in 

 contrast to the heterogeneous results under 

 less containing pressures where competency 

 and strength of rocks play a part. 



Earth temperatures increase with depth. 

 Increase in temperature aids and accelerates 

 rock flowage. This is evidenced by ilowage of 

 hard, rocks at moderate depths at batholithic 

 contacts. Also facts of physical chemistry 

 show that increase of temperature increases 

 molecular activity, hastens endothermic re- 

 actions (anamorphic reactions are largely 

 endothermic), increases solution, both liquid 

 and solid, and hence recrystallization, and de- 

 creases viscosity or internal friction. 



Notwithstanding these and other considera- 

 tions, any conclusions as to the existence of 

 a deep zone in which all rocks flow when de- 

 formed is hypothesis, not proved fact, and 

 perhaps will always remain so. The environ- 

 mental conditions are not accurately known; 

 and even if each of the factors were measured, 

 their conjoint effect is still speculative. Vari- 

 ations in the time factor alone may determine 

 whether a rock flows or fractures. Eock flow- 

 age which has occurred in rocks now ac- 

 cessible to our observation fails to indicate 

 increase with depth with sufficient clearness 

 and definiteness to warrant confident down- 

 ward projection. 



Experimental evidence has been construed 

 to indicate that under great containing pres- 

 sures, of the kind probably existing at depth, 

 the movement under thi'ust or shear is of 

 the nature of rock flowage, but this is partly 

 a matter of definition. The rock breaks and 

 granulates, often along definite planes, but 

 the parts are still held together; it really 

 flows. Displacements along these planes may 

 partake of the nature of faults, and there is 

 no development of true flow cleavage deter- 

 mined by a parallel arrangement of minerals 

 under recrystallization, the common geologic 



evidence of rock flow. Presumably with 

 longer time and proper conditions of tempera- 

 ture and mineralizers, parallelism of newly 

 developed minerals, characteristic of rock 

 flow, would result. So far as the experimental 

 results go, however, they fail to exhibit struc- 

 tures which in ordinary geologic field inter- 

 pretation would be classed as typical rock 

 flowage. They would be called fracture or 

 combined fracture and flowage. They would 

 be described as shear planes and faults. They 

 might suggest rupture of the kind that origi- 

 nates earthquake shocks. 



Eock flowage has been widely assumed to 

 indicate weakness and mobility. The correla- 

 tion of rock flowage with weakness may arise 

 from the fact that certain soft rocks such as 

 shales, which are inherently weak, may often 

 be observed to have undergone rock flowage, 

 while adjacent strong rocks have been im- 

 affected. Or, a zone of flowage passing 

 through a homogeneous formation unques- 

 tionably indicates movement along the flowage 

 zon«, and, therefore, indicates the weakness of 

 this zone relative to adjacent undeformed 

 parts of the mass. But it would be equally 

 valid to argue that where fracturing has been 

 concentrated along a zone between unde- 

 formed rocks it too indicates movement, and 

 therefore relative weakness. It is a long step 

 from this to the conclusion that flowage indi- 

 cates greater weakness than fracture. It is 

 entirely conceivable that it might require 

 more energy to malve rock flow than to make 

 it fracture. Indeed there is some reason for 

 believing, both from experimental work and 

 from observations in areas of combined frac- 

 ture and flowage, that relief actually takes 

 place first and most easily by fracture and 

 that fiowage occurs only when it is possible 

 to concentrate much more energy into the 

 rock. Both structures show weakness relative 

 to adjacent undeformed masses, but in rela- 

 tion to each other degree of weakness is a 

 much more complicated problem. 



Our question, then, as to the extent to 

 which deep movements are accomplished by 

 rock flowage can not be simply and definitely 

 answered in the present state of knowledge. 



