38 



NEW YORK STATE MUSEUM 



surrounding garnet gneiss bends round them closely in such a man- 

 ner that the whole phenomenon resembles a partially melted xeno- 

 lith in a viscous and flowing magma (see plate 6, upper figure). 

 This is the feature referred to earlier as being of pseudo-igneous 

 nature (page ^5)- It is not necessan,', however, nor safe, to base 

 an hypothesis of the igneous origin of the garnet gneiss upon 

 this phenomenon alone, for it seems possible to account for it on 

 purely dynamic grounds. The strips of amphibolite are best 

 regarded as having been intruded into the garnet gneiss, sill- 

 fashion, at different horizons, in masses var}-ing from a few inches 

 (see plate 6, lower figure) to 20 or more feet in thickness. These 

 were afterward broken during the period of crustal disturbance 

 which resulted in the formation of the sigmoid flexure, and the 

 garnet gneiss was squeezed in betsveen the separated fragments. 

 That the amphibolites yielded to the deformative pressure in nearly 

 the same degree as the garnet gneiss is evidenced by the fact that 

 they are seldom broken squarely across, nor do they exhibit angular 

 boundaries: on the other hand, in the few places where the ends 

 of tlie masses are seen, they taper av\-ay lenslike, though generally 

 with some abruptness. The garnet gneiss, however, possessed a 

 degree of viscosit}* less than that of the amphibolites, and because 

 of this relative fluidit}* wholly inclosed the parts of the disarticu- 

 lated sills, and assmned toward them in this respect a role resemb- 

 ling that of a viscous igneous magma. 



That the garnet gneisses, as a group, possess the capacit}- for 

 minute deformation without rupture, is shown independently at other 



Fig. 6 Detached fragment of cn.'^taiiine limestone within a garaet-gneis? 

 xenolith. A thin limestone stratum has been pulled apart and one of the 

 fragments pinched off at the ends. The structure lines in the gneiss roughly 

 follow the boundaries of the detached pieces, (a) Garnet gneiss; (b) t^-pical 

 Grenville limestone; (<:) surficial covering. 



About 1.6 miles southeast of Langdon Comer?, in Pierrepont. 



