FOYAITE-IJOLITE SERIES OF MAGNET COVE 655 



United States Geological Survey Reference Collection (No. 813). 

 It is of a small dike, about one and a quarter inches wide, cutting 

 the shale near Neusch's gulley, which it has metamorphosed. 

 I am indebted to Dr. Ransome for the examination and descrip- 

 tion of this specimen which he sent at my request. "Under the 

 microscope, the dikelet is seen to consist almost wholly of cloudy 

 alkali-feldspar, with no quartz or nephelite, and a little biotite. 

 With high power, the feldspar (between crossed nicols) all shows 

 the fine shadowy striping indicative of a soda-bearing feldspar." 

 Two garnets also occur at the borders of the dike. The occur- 

 rence of this aplite dike is clearly corroborative of the view of 

 the course of differentiation which has been just expressed, and 

 it is probable that further search would reveal others which have 

 heretofore escaped notice. 



Turning to the other end of the diagram, there is good 

 ground for the belief that there must be inflexion upwards of 

 one or more of the curves beyond II to the left. If the curves 

 are extrapolated to the left, at a distance, let us say, equal to 

 that between II and III, the sum of the constituents reduced to 

 precentages amounts to only 55.6. 



It is obvious therefore, either that some other component of 

 the magma than any of those plotted is greatly concentrated at 

 the basic end, or else that the curves of one or more of the 

 plotted constituents must run very sharply upward, thus causing 

 inflexion. 



In the former case a probable additional constituent would 

 be PgOg, which would yield, with high CaO, MgO and (FeO), 

 an apatite-rich pyroxenite like that of Ahvenvaara in Finland,^ 

 or with disappearance of SiOg, an apatite-magnetite rock like 

 that of Alno.'' If TiOg should be the constituent to assume 

 extraordinary proportions toward the basic end, we would expect, 

 with disappearance of SiO^, titaniferous magnetites, or such rocks 

 as the magnetite-perofskite rock of Brazil, described by Derby. ^ 



'V. Hackman, Bull. Com. Geol. FiiiL, No. XI, p. 36, 1900. 

 '^ Cf. RosENBUSCH, Elemente, No. 3, p. 133, 189S. 

 30. A. Derby. Neiies Jahrb., 1894, Vol. II, p. 297. 



