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between these two minerals is to etch the slide with acid in the 

 manner to be presently described. 



Plagioclase. In the elseolite-syenites of Norway this mineral is 

 rare. In other rocks of the same general character it is more 

 abundant. It shows extremely fine twin-striation and appears to be 

 allied to oligoclase or albite. Basic felspars are not known in these 

 rocks. The plagioclase of the other rocks belonging to the group 

 now under consideration calls for no special description. 



Elceolite and Nepheline. The only distinction between these two 

 minerals depends upon their mode of occurrence and general aspect. 

 Elseolite is characteristic of the plutonic rocks and possesses a dull 

 lustre. It is also often somewhat coloured (greenish, brownish or 

 grey). Nepheline is characteristic of the volcanic rocks and possesses 

 a glassy lustre. In the elseolite-syenites the elseolite often occurs in 

 irregular grains. When crystalline form is developed the faces are 

 the same as those of nepheline. By alteration it gives rise to zeolites, 

 such as anaclime and thomsonite, or to cancrinite. The optical 

 characters of elseolite are the same as those of nepheline. Nepheline 

 is especially characteristic of the trachytic rocks. Idiomorphic crystals 

 show the faces of the prism (1010) and basal plane (0001) ; less 

 frequent also those of the pyramid (1011). Sections at right angles 

 to the morphological axis are hexagonal, those parallel to this axis 

 are rectangular or lath-shaped. When the pyramid is present the 

 right-angles are truncated. The thickness of the crystal measured at 

 right-angles to the morphological axis is generally equal to or 

 greater than the thickness measured in the direction of this axis. 

 Thus in the lath-shaped sections the short side lies parallel to the 

 principal axis. This fact must be borne in mind in determining the 

 character of the double-refraction of a doubtful mineral by observation 

 with the quartz-wedge. The refraction and double-refraction are both 

 low. In sections of the ordinary thickness nepheline gives the grey- 

 blue and white tints of the first-order. Sections at right-angles to 

 the optic axis give, in convergent polarized light, a broad indistinct 

 cross in consequence of the low double-refraction of the mineral. As 

 a general rule nepheline does not occur in large porphyritic crystals 

 but only as a constituent of the ground-mass. Its recognition is often 

 attended with great difficulty : especially when, as sometimes happens, 

 it plays the role of ground-mass and does not show definite form. 

 To make sure of nepheline and to separate it from other minerals it 

 is frequently necessary to treat the slide with acid. (1) Nepheline may 



(1) This should be done as follows : Uncover a portion of the section and remove the 

 canada-balsam with alcohol. Place a drop of hydrochloric acid on the uncovered section and 

 spread it evenly over the slide ; heat gently. Wash off the acid and then place on the slide a 

 solution of fuchsin or some other material suitable for staining gelatinous silica. Wash with 

 water and observe under the microscope. If everything has gone well the nepheline, and other 

 minerals decomposable with separation of gelatinous silica, will be distinctly recognizable by the 

 distribution of the staining substance. If the appearance after one treatment is unsatisfactory 

 the process must be repeated. If too thick a coating of acid be used the gelatinous silica, will 

 spread out over the minerals not affected, and the result will be unsatisfactory. 



