214 ANNALS NEW YORK ACADEMY OF SCIENCES 



underlying limestone also agrees with such a view. The gradation of the 

 limestone into schist and the occurrence of thin beds of schist in the lime- 

 stone near the contact is what one would expect to find when the condi- 

 tions favorable for the deposition of a limestone gradually changed toward 

 those leading to the deposition of an argillaceous sediment. Evidently 

 the two formations are conformable. 



Later, these strata underwent profound regional metamorphism which 

 led to the complete recrystallization of the constituents present. These 

 changes were brought about through burial to a considerable depth under- 

 neath other sediments, followed by the inauguration of a period of great 

 orogenic movements which brought about the intense folding of the strata 

 involved. These orogenic movements were accompanied by a series of 

 granitic intrusives which are described later and which also must have 

 been important factors leading toward the thorough recrystallization of 

 the original sediments. Their effect will be discussed in somewhat greater 

 detail in another paragraph. 



The formation of the schist, therefore, took place under mass-mechan- 

 ical conditions in the zone of ana-morphism, as described by Professor 

 Van Hise.^^ If we follow Dr. Grubenmann's plan of dividing the outer 

 portion of the earth into three zones, based upon the nature of the meta- 

 mo77)hic changes taking place at different depths, the formation of the 

 schist took place in the middle zone. In this zone, as described by Dr. 

 Grubenmann,^^ the temperature is notably higher than in the upper zone, 

 and pressure and temperature alike tend to work toward the production 

 of such minerals as represent the smallest molecular volumes and highest 

 specific gravities for the constituent components present. The pressure 

 is mostly due to stress, although hydrostatic pressure due to the superin- 

 cumbent mass also begins to become eifected. There is little possibility 

 of any movement of the particles, and stress aided by temperature, there- 

 fore, works principally toward recr}^stallization, so that chemical action 

 not only keeps pace with mechanical but even exceeds it. Wholly crystal- 

 line rocks are therefore formed in this zone, and good cataclastic structures 

 are not of common occurrence. On account of the fact that the prevailing 

 pressure is due to stress, this is the home of the schists. Tlie character- 

 istic minerals of this zone are muscovite, biotite, zoisite, epidote and to a 

 lesser extent hornblende, staurolite, garnet and cyanite. Dr. Gruben- 

 mann^^ also calls attention to the well known fact that the higher the 

 temperature and pressure under these conditions the greater will be the 



23 Monograph XLVII. U. S. Geol. Surv., pp. 685-698. 1904. 

 3* Die kristallinen Schiefer. Zweite Auflage, p. 78. 1910. 

 ■^ Ibid., p. 75. 



