278 HAROLD L. ALLING 
but this may not be a safe criterion, in that quartz may have been 
reorganized into meta- and trisilicates. 
Seeking for a more reliable distinction the pyroxene-amphibole 
(the pyribole of Johannsen)' content was examined. It is held by 
many geochemists? that pyroxene is a high-temperature mineral, 
while amphibole is a lower-temperature form. The change from 
one to the other being a paramorphic (or ‘“‘autometamorphic”’) one 
—a change readily brought about by the stresses of dynamic and 
static metamorphism—the inversion of pyroxene to amphibole 
furnishes some aid in the problems in hand. If a large amount of 
pyroxene, such as augite, is found in an amphibolite it suggests an 
igneous origin. But under the stress of severe metamorphism this 
inversion may be complete. Martin’ found this to be true of the 
amphibolite inclusions in the granitic rocks in St. Lawrence County. 
Thus the absence of augite does not prove a sedimentary parentage, 
but merely suggests it. This criterion, like the former, is therefore 
regarded as inconclusive. 
Hunting for additional criteria, the writer investigated the feldspars in 
turn. It was found that the igneous types usually contained a simple range 
of feldspars, such as ro per cent of soda orthoclase and 20 per cent of andesine, 
while the sedimentary rocks frequently exhibited a motley collection; cover- 
ing a much wider range. Very commonly soda orthoclase, soda microcline, 
microperthite, oligoclase, and labradorite were seen in a single microscopic slide.4 
Adams’ states that the amphibolite occurring near Jack Lake, 
Ontario, to which an igneous origin must be ascribed, is “composed 
almost exclusively of hornblende, and plagioclase feldspar. The 
hornblende is rather light green in color in ordinary light... . . 
The plagioclase is clear and fresh in appearance, and rather basic 
t Albert Johannsen, Jour. Geol., XIX, p. 319, I9II. 
2J. V. Elsden, Principles of Chemical Geology, p. 114, 1910; Becke, Tsch. Mineral 
und Petrog. Mitt., 16, pp. 327-36; F. W. Clarke, U.S. Geol. Surv. Bull. 616, p. 386; 
Lacroix, Mineralogie de la France, I (1893-95), pp. 668-69. 
3 J. C. Martin, New York State Mus. Bull. 185 (1916), p. 157. 
4H. L. Alling, Amer. Jour. Sci. (4), XLVIII (1919), pp. 61-62. 
5F. D. Adams, “On the Origin of the Amphibolites of the Laurentian Area of 
Canada,” Jour. Geol., XVII (1909), pp. 1-18; F. D. Adams and A. E. Barlow, “ Geology 
of the Halburton and Bancroft Areas,” Province of Ontario, Canada, Dept. of 
Mines, Geol. Survey Branch, Mem. 6 (1910), pp. 160-61. 
