THIRD HYPOTHESIS OF DERIVATION 479 



distinctness in the. minor layers. I have explained this on the ground that horn- 

 hlende is relatively a fusible mineral (being of the grade 3, on von Kobell's scale of 

 fusibility), while the feldspar (of which orthoclase is the prevailing one) and the 

 mica (black and white) are of difficult fusibility (5 to 6, on the same scale), and, 

 in consequence, beds that become hornblendic in the metamorphic process easily 

 soften and bend. Such observations show why hornblendic metamorphic rocks, 

 when containing little or no quartz, often fail of bedding, and look enough like 

 igneous rocks to be frequently referred, without a question, to that class." 



But thermal conditions concerned in metamorphism of schists, it is 

 w^ell established, have ruled far below the fusion point of hornblende 

 (1060 degrees centigrade, according to Alb. Brun*), being generally es- 

 timated at a temperature of 150 to 650 degrees centigrade in presence of 

 moisture. The plasticity therefore indicated by these corrugations must 

 be due to some other cause. Subordinate conditions, apparently, have 

 been the condensation suffered by the original coarse trap during its 

 shearing into thinly laminated schist, as well as the softening and mo- 

 bility promoted by the action of heated waters and mineralizers, shown 

 by close and constant association with masses of pegmatite. 



Yet more efficient has been the peculiar texture of the hornblende 

 rock, made up of polished blades and plates, intermixed with slippery 

 scales of hematite and biotite, the latter often developed and concen- 

 trated in continuous micaceous films. Under the intense pressures in- 

 volved in mashing and folding of the Manhattan stratum, great differ- 

 ences prevailed in rigidity of different parts, varying from sandstone-like 

 beds of compact quartzose gneiss, coarsely foliated micaceous gneiss, 

 more fissile mica schist, and occasional thin sheets of trap. Both in 

 these sheets and in the pliant micaceous layers adjoining we find all the 

 properties to favor easy fiowage during orogenic movements, with ready 

 tendency to extreme variations in thickness, flexure, and distortion. As 

 the general process has been explained : 



" In a series or group of beds of different lithological character, the thick strong 

 beds are less closely folded than the thin weak beds. The softer layers are greatly 

 thickened here and greatly thinned there, as demanded by the stronger Tayers. 

 The folding of the first may be comparatively simple and the second may be closely 

 plicated." f 



To this general action might have been added in certain cases, as sug- 

 gested in a previous connection, the downward crumpling of the upright 

 portions of the ancient dikes. 



Again, in regard to the evidences of rigidit3^ brittleness, and fragility 

 of the schist, shown by its fracture, faulting, and occasional brecciation, 



*Arch. de Sci. phys. et nat., xiii, p. 352. 

 t Van Hise, U. S. Geol. Survey, Ann. Rep., vol. xvi, pt. 1, 1894-'95, p. 596. 



