THE ORIGIN OF FOLIATED CRYSTALLINE ROCKS 231 
most recently been described by Loughlint in Connecticut and 
Rogers’ in the state of New York. The following have been sug- 
gested as criteria for distinguishing these gneisses from those 
formed by the alteration of solid igneous rocks: 
Field evidence: Banding in apophyses from the gneiss parallel 
to the walls and at an angle to the schistosity of the inclosing 
rock,’ dikes of pegmatite belonging to the same magmatic series 
as the gneiss and either parallel to the gneissic structure and 
foliated with it or cutting the gneissic structure and undisturbed; 
lack of sharp contact between the acidic and more basic portions of 
the gneiss, indicating high temperature during the solidifications of 
the different bands; presence of inclusions of foreign rock, which 
are but slightly deformed, in a matrix of well-banded gneiss; 
presence of distinct bands of widely different composition, none 
of which may show evidence of shearing; flowlike curves of the 
banding, some of which may close in a circle. 
Mineralogical evidence: Presence of minerals formed char- 
acteristically only from igneous melts and arranged in a manner 
impossible of formation from solid rocks by metamorphism, e.g., 
nepheline and olivine; textures due to crystallization from an 
igneous melt. Weinschenk® considers that epidote, garnet clino- 
zoicite, sillimanite, and chlorite crystallize from the magma in the 
case of primary gneisses on account of the pressure present during 
the solidification of the rock, but the exact state of the rock during 
their formation is not definitely known. 
SCOPE OF PAPER 
In the following thesis only three of the many criteria which 
have been proposed have been considered. They are (1) the 
criterion of texture as applied to primary gneisses, (2) uses of 
zircon as a criterion, (3) use of chemical composition in the deter- 
«1G. F. Loughlin, Am. Jour. Sci., 4th Ser., X XTX (1910), 447-56. 
2G. S. Rogers, Am. Jour. Sci., 4th Ser., XX XI (1911), 125-30. 
3 J. W. Gregory, Q.J. Geol. Soc., L (1894), 265. 
4 Geol. Surv. Can., Mem. 6 (1910), 83. 
5 Geol. Mag., N.S., Decade 4, IV (1897), 354- 
6 Congrés géol. inter., compte rendu, session VIII, I (1900), 340. 
