186 J. Barrell — Subjacent Igneous Invasions. 



batholithic invasion. How, then, shall the geologic evi- 

 dence be reconciled with the laboratory demonstrations 

 of Adams! This is not a conflict between facts and an 

 imperfect theory, but rather between two categories of 

 facts. Experimental work as conducted by Adams re- 

 quires temperatures and pressures for rock flowage far in 

 excess of the conditions implied by the geologic evidence. 

 It is the business of theory to explain the paradox. 



The theory of rock flowage, in the form stated by Van 

 Hise, calls for an intermediate level. What adjustments 

 of this theory are needed to bring harmony between it and 

 the apparently diverging facts ? It appears that the most 

 probable solution may be found by invoking the high tem- 

 peratures and abundant crystallizers which must be 

 present in batholithic roofs. ^^ Let crustal compression 

 operate at the same time and stress-differences arise 

 through the crust. But the molecules under strain are 

 more soluble, energy is released by their solution, and 

 they are precipitated free from strain. The energy 

 absorbed in deformation is transformed into heat and 

 assists further in the process of recrystallization. 



The phenomenon of rock granulation is suggestive of 

 a very considerable depth of cover, yet even for this the 

 geologic evidence does not support the view that any such 

 profound depth is necessary as the laboratory work of 

 Adams would seem to imply. It occurs especially in the 

 granite gneisses, massive and resistant formations. In 

 part it seems to be related to a last stage in crystallization 

 accompanied by movements in a very viscous magma. 

 The granulation of phenocrysts in rhyolite surface flows 

 offers a suggestion of the process. In part the presence 

 of foliation at right angles to the pressure may prevent 

 shearing on major diagonal thrust planes and compel 

 each layer to yield independently by intramineral shear. 



(To be continued) 



^*Daly has presented, in a paper on metamorphism in volume 28 of the 

 Bulletin of the Geological Society of America, a similar argument that the 

 depth of pre-Cambrian metamorphism Nvas not great; he suggests that it 

 is to be explained by a steeper thermal gradient in the early history of the 

 earth. This is one factor in the present suggestion, the other being emana- 

 tions from the same magma that produced the steeper thermal gradient. 



