^2 NEW YORK STATE MUSEUM 



Perhaps the most significant points of mineraloglcal difference 

 between the granite of the sigmoid area and those larger masses in 

 the western half of the quadrangle, relate to or depend upon its role 

 as an injection rock. The intimate character of this admixture has 

 already been referred to, but the nature of the resulting mixed rocks 

 remains to be indicated. The coarse-textured or leaf type of injec- 

 tion is the dominant type throughout this vicinity, and accordingly 

 the specific characteristics of each of the constituent rock units are 

 little if at all affected by the interbanding. On the other hand, in 

 those cases where the mixture is of a finer grade, the components 

 of the injection gneiss have been physically or chemically blended 

 and have produced a hybrid rock of a composition in all cases inter- 

 mediate between the granite and the gabbro-diorite. The end prod- 

 uct itself varies, however, according to the proportion of acid and 

 basic material entering into the mixed rock, that is, the degree of 

 assimilation, and according to gradations in the amount of meta- 

 morphism previously undergone by the basic component. Such 

 variations can be obser\^ed at different points in the outer injection 

 zone of the eastern half of the sigmoid. They grade from horn- 

 blende-bearing granite, through grano-diorite to augite-syenite, and 

 gabbro-diorite. 



Different steps in this process of assimilation are illustrated in 

 plates II, lower figure, and 12, lower figure, of which the former is 

 a hornblende-granite and the latter an augite-syenite or diorite, both 

 of which have suffered intense subsequent granulation. In the first 

 case the components are a large quantity of granite material and a 

 small quantity of amphibolite whose disintegration by the granite 

 has been complete ; in the second, the basic rock preponderates, and 

 appears to have been of gabbro-dioritic composition and not of the 

 more usual, completely m.etamorphosed amphibolitic character. 

 Gradual variations in the mineralogic character of these mixed 

 rocks between the limits named, produce an abundance of types. 

 It is not thought necessaiy, however, to attempt to describe or even 

 to list these in detail, inasmuch as the two examples given serve to 

 illustrate the essential nature and cause of such variations. 



As to the method of the assimilation process, it is possible that 

 two distinct factors, of chemical and physical nature respectively, 

 have operated in the production of the final mixed rocks. It is im- 

 probable that the whole composite zone of Grenville garnet gneiss 

 and intruded gabbro-amphibolite into which the granite was in- 

 jected, was made fluid by the heat of intrusion; but small portions 



