2?4 F. D. Adams — New Nepheline Rock. 



tion is treated with acid and etched, however, the plagioclase 

 is seen to occur in individuals of a more or less rounded form 

 or with curving outlines, lying between the nepheline grains 

 or enclosed in the latter. The feldspar isolated from another 

 variety of the rock in the same occurrence was found to be 

 albite, and this feldspar has, therefore, been taken as albite in 

 calculating the Mode of the rock. 



The amount of Cancrinite present varies very considerably 

 in different specimens of the rock. In the specimen analyzed 

 about 5 per cent was found. In other specimens more is found, 

 although in no case is it very abundant. It is clear and color- 

 less, but is at once distinguished from the nepheline when 

 examined between crossed nicols by its much higher polariza- 

 tion colors, which in thin sections frequently rise to a blue of 

 the second order. It is clear and free from interpositions and 

 in convergent light is seen to be uniaxial and negative. It also 

 shows a slight but distinct dispersion of the bisectrices, giving 

 a brownish and a bluish tint on either side of the position of 

 maximum extinction. When separated by Thoulet's solution, 

 the mineral was found to have a specific gravity between 2*48 

 and 2*44, and to be readily decomposed when heated with dilute 

 hydrochloric acid with the evolution of carbonic dioxide and 

 with subsequent gelatinization. The cancrinite occurs in the 

 nepheline in the form of narrow strings or more rarely in little 

 bunches of grains. These usually follow the course of minute 

 cracks or cleavage lines, but also are frequently seen to follow 

 the boundaries of individual grains of nepheline on their contact 

 with grains of other minerals. Thus between crossed nicols 

 they appear as a brilliant edging about hornblende individu- 

 als or about calcite inclusions in the nepheline, the small pris- 

 matic individuals of cancrinite being arranged with their longer 

 axes at right angles to the contact or to the course of the crack, 

 as the case may be. The cancrinite has the appearance of 

 being an alteration product of the nepheline. 



The Calcite occurs in large single individuals, which are found 

 as inclusions in both the hornblende and the nepheline. The 

 single individuals are often perfectly circular in outline, and the 

 enclosing mineral is perfectly fresh and unaltered and is sharply 

 defined against them. In other cases the same large calcite 

 individuals lie between the other constituents of the rock, in 

 all cases having the character of inclusions. They generally 

 show very marked strain shadows, while the other constituents 

 show but little or no evidence of pressure phenomena. 



The Apatite is found as occasional more or less rounded indi- 

 viduals, enclosed in the nepheline or hornblende, but, like the 

 other accessory constituents, merits no especial description. 



An analysis of the rock made for me by Mr. M. F. Connor 

 gave the following results : 



