230 J. D. TRUEMAN 
decomposed during sedimentation, such as nepheline,’ leucite, 
etc.; presence of unaltered minerals characteristically formed 
only from igneous melts and which become modified or segregated 
during sedimentation, such as zircon and monazite; presence of 
secondary minerals generally considered to be more characteristic 
of altered igneous than altered sedimentary rocks, such as epidote, 
zoicite, chlorite, and hornblende.? For sedimentary origin—pres- 
ervation of original fragmental texture; presence of secondary 
minerals supposedly more characteristic of altered sedimentary 
than altered igneous rocks, e.g., a group high in AL,O, and low in 
bases, such as staurolite, andalusite, sillimanite, and cyanite, but. 
also other minerals as biotite, garnet, and graphite. 
Chemical evidence: For sedimentary origin—variation from 
normal igneous rock types as shown by comparison with classi- 
fied tables of igneous rocks arranged according to chemical composi- 
tion; molecular ratio of Al,O, to Na,O, K,O and CaO greater than 
1; excess of K,O over Na,O by weight; excess of MgO over CaO 
by weight;4 high AI,O, content; high SiO, content. 
Criteria for distinguishing primary gneisses from metamorphic 
rocks with a banded structure—Many criteria have been suggested 
for the recognition of igneous rocks whose foliation was produced 
during the consolidation of the rock. The importance of this 
rock class has not been conceded by all geologists, though gneisses 
have been confidently described as such by Lawson,’ Geikie and 
Teall,© Bonney,’ Barlow,’ McMahon,? Weinschenk,’ Adams and 
Barlow, and many others. Examples of primary gneisses have 
«W. H. Emmons, ‘‘A Genetic Classification of Minerals,’ Econ. Geol., III (1908), 
620. 
2C. R. Van Hise, “Treatise on Metamorphism,” U.S.G.S., Mono. XLVII 
(1904), 916. 
3F. Bascom, Geol. Soc. Amer. Bull., XVI (1905), 294-95. 
4E.S. Bastin, Jour. Geol., XVII (1909), 445. 
5 A.C. Lawson, Ann. Rep. Geol. Surv. Can., N.S. III, (1887), 139 f. 
6 A. Geikie and J. J. H. Teall, Quar. Jour. Geol. Soc., L (1894), 645. 
7T. G. Bonney, Q./J. Geol. Soc., LIL (1896), 17. 
8 A. E. Barlow, Aun. Rep. Geol. Surv. Can., N.S., X, Part 1 (1897), 48-87. 
9 C. A. McMahon, Geol. Mag., N.S., Decade 4, IV (1897), 345-55. 
10 —. Weinschenk, Congrés géol. inter., compte rendu, session VIII, I (1900), 326-40. 
1 F, D—. Adams and A. E. Barlow, Geol. Surv. Can., Mem. 6 (1910), 83. 
