438 JOSEPH BARRELL 



the students of anamorphism, the process has depended upon a 

 readier solution of the molecules under shearing stress than of those 

 free from such stress — solution carried on by means of the relatively 

 minute proportion of gaseous crystallizers which were present 

 through the rock mass. Such crystalKzers doubtless facilitate the 

 process. They form, in fact, solutions with the rock which may be 

 regarded as mixtures with very low fusion points. But, theoretic- 

 ally, as the temperatures approach those of general fusion the need 

 of such crystallizers diminishes. Moderate shearing stresses can 

 thus liquefy the parts upon which they act and a process analogous 

 to glacial motion sets in for soHd rock. The strength of highly 

 heated rock appears then dependent upon the amount by which the 

 temperature is below the melting-point. That zone of the earth 

 which is very weak may then be regarded as approximately at the 

 temperature of fusion. To transform the solid into liquid there is 

 needed only the energy required for latent heat and increase of 

 volume. The proportion of liquid which is generated will vary 

 directly with the amount of heat supplied or the amount of hydro- 

 static pressure removed. Magma, consequently, can be generated 

 in this zone more readily than above in the zone of strength; but 

 it will not be in reservoirs; rather will it be in its place of origin 

 disseminated through the rock mass like water standing in a porous 

 sandstone.^ 



ANALOGIES BETWEEN ASTHENOSPHERIC ROCK AND GLACIAL ICE 



The theory of the asthenosphere as here presented is seen to 

 have important relations to other branches of geology. The zone 

 of weakness becomes especially the generator of magmas; the 



^ Recently the writer has learned from Mr. Bailey Willis of an unpublished paper 

 which he gave some years ago to the Geological Society of Washington, in which he 

 outlined his views of the nature of crustal thrusts as illustrated by the Appalachians. 

 In that, and more recently, as a result of studies in the Alps and Andes, he has come to 

 entertain the view that the zone of compensation, the lithosphere, shears over the zone 

 below through the agency of molecular or mass fusion. Deep-seated horizontal shear 

 and igneous intrusion he thus holds have important associations with orogenic move- 

 ments. We have thus arrived independently at somewhat the same view of the 

 nature of the zone of weakness. The part which recrystallization may play in pro- 

 moting such movement is suggested in his Research in China, Vol. II (1907), "Syste- 

 matic Geology," pp. 130, 131. 



