48 DE. J. W. EVANS ON A MONCIUQUITE FROM [Feb. I90I, 



is included, and '62 of the water was given off over sulphuric 

 acid. 



No. Y is the mean of eighteen analyses of analcime from the 6th 

 ed. of Dana's ' System of Mineralogy ' (1 to 19, but omitting 3). 



Finally, Nos. VI & YII represent the theoretical composition of 

 analcime and nepheline respectively. 



The similarity of the figures for analcime to those of Nos. 1 6, II b, 

 III, & IV is very striking. 



The foregoing comparison of analyses is sufficient to show that 

 the composition of the isotropic material in the Girnar 

 rock approximates closely to that of analcime. 



V. The JN'atfre of the Isoteopig GeoUxNdmass. 



VVe can have little hesitation in coming to the conclusion that 

 much, if not all the isotropic material in the rock which has been 

 dealt with in the present paper consists simply of the mineral 

 analcime. Like that substance, it is isotropic ; has a low index of 

 refraction ; gelatinizes with acid ; its specific gravity is about 2*2 ; 

 and it has the same chemical composition. Yet it might have all 

 these characters in common with analcime, and nevertheless be 

 a structureless glass. But the arrangement and orientation of its 

 inclusions, and the distinct traces of cleavage which are visible, 

 show that it is crystalline in structure. Being crystalline, its 

 isotropic character naturally places it in the cubic system ; and 

 with this determination the character of the symmetry indicated 

 by the cleavage and the majority of the inclusions is in no way 

 inconsistent.^ 



It is remarkable that in this occurrence of the mineral there 

 should be rarely, if ever, any trace of a crystalline boundary ; but 

 this will be explained, if we remember that the analcime must have 

 been the last mineral to form, and is therefore naturally allotrio- 

 morphic. It is more difficult to understand the complete absence 

 of anomalous double-refraction, manifested by faint luminous bands 

 under crossed nicols ; but this exceptional behaviour cannot be con- 

 sidered a sufficient ground for refusing to accept the identification 

 with analcime — a determination which is supported by the fact 

 that a similar isotropic substance is found filling the interstices 

 between the minerals of a rock at a distance of about 1| miles, 

 near the foot of the western slope of the central ridge of Girnar, 

 a few yards south of the carriage-road. Here the grain of the rock 

 is much coarser, and the presence of glass seems in the hiiihest degree 

 improbable, (I hope to give a detailed description of this rock at 

 an early date.) 



It is important to observe that the occurrence of an analcime- 

 matrix implies the survival of a magma of the same composition after 



^ Some of the inclusions appear to be parallel to the edges of the cube, and 

 there is reason to suppose that others are parallel to the edges of the eikosi- 

 tetrahedron (trapezoidal triakis-octahedron) in which analcime crys- 

 tallizes. 



