598 WILLIAM J. MILLER 



mendous erosion when at least some miles, and quite possibly 

 many miles, in thickness of materials were removed. Thus it 

 seems clear that much of the Grenville rock now visible was once 

 far more deeply buried than any known body of sediments since 

 the beginning of the Paleozoic. Conditions of downward pressure 

 and temperature were, therefore, more than usually favorable for 

 static metamorphism. On the basis of static metamorphism it 

 is not necessary to account for a high degree of metamorphism, 

 because the Grenville series, though thoroughly crystalline, is 

 mostly only moderately foliated with relatively little granulation, 

 and with stratification generally well preserved. It may also be 

 suggested as a possibility that actual crystallization did not begin 

 until an early stage in the intrusion of the slowly upwelling magmas 

 when additional heat for regional metamorphism was supplied. 



Evidence from other sources. — Experimental evidence is also 

 suggestive in this connection. Thus, Becker and Day 1 have proved 

 that crystals in general have a strong tendency to grow (or elongate) 

 most rapidly at right angles to the direction of pressure. According 

 to Wright, 2 cubes of glass formed by melting together wollastonite, 

 diopside, and anorthite heated to the state of incipient crystalliza- 

 tion under vertical pressure, showed, under the microscope, that 

 the three minerals crystallized with long axes at right angles 

 to the direction of pressure. Experimental evidence, therefore, 

 strongly supports the possible development of elongated crystals 

 in the Grenville sediments under conditions of static metamorphism. 



Van Hise has suggested, regarding the parallelism of foliation 

 and bedding in the Grenville series, that "vertical shortening and 

 consequently horizontal elongation below the level of no lateral 

 stress may have begun the process." 3 The writer views this as 

 essentially the whole process, instead of assuming, as Van Hise did, 

 that foliation parallel to bedding continued to develop under certain 

 peculiar conditions when the rocks were subsequently folded. 

 The explanation of foliation parallel to bedding is greatly simplified 

 when it is not necessary to consider severe compression of the region. 



1 Becker and Day, Proc. Wash. Acad. Sci., VII (1905), 283-88. 



2 F. E. Wright, Am. Jour. Sci., 4th series, XXII (1906), 226. 



3 C. R. Van Hise, U.S. Geol. Surv., 16th Ann. Rep., Part I, p. 773. 



