F. D. Adams — Melilite-bearing Rock. 277 



Melilite. — This mineral which gives to the rock its distinc- 

 tive character is well characterized and is quite abundant in 

 some of the sections, while in others it occurs more sparingly. 

 It is found almost invaribly as flat tabular crystals shortened m 

 the direction of the vertical axis. Basal sections of these crys- 

 tals have rectangular or eight sided outlines and are isotropic, 

 while prismatic sections present lath shaped forms which 

 between crossed nicols show parallel extinction. Sometimes 

 however the crystals are not so flattened but have the form of 

 short stout prisms while frequently they are irregular in shape. 

 The individuals ordinarily range in size from -GIG 111 " 1 to •046 mm 

 in thickness and from -07 mm to -17 mm in length. The largest of 

 a considerable number which were measured was -46 mm by 

 *057 mm , this however is unusually large. Tornebohm says that 

 in the rock from Alno the melilite crystals are seldom over 'l mm 

 in length. The mineral is usually colorless but often has a slight 

 but distinct yellowish tint. It has a much higher index of 

 refraction than nepheline but the double refraction is weak, 

 the polarization colors in good sections varying from very dull 

 blue to bluish gray. In the lath shaped sections a basal cleav- 

 age can often be distinctly seen and in such cases the extinction 

 is of course parallel to it, as it is parallel to the sides of the lath 

 shaped sections when the cleavage is not seen. At right angles 

 to this cleavage and therefore parallel to the vertical axis, what 

 Stelzner has called " peg structure," which is so characteristic 

 of melilite is often excellently developed. This cleavage with 

 the " peg structure " at right angles to it, when well seen in- 

 dicates with certainty the orientation of the melilite individual 

 and the study of a large number of such grains in slides pre- 

 pared from several hand specimens, with the aid of a gypsum 

 film giving between crossed nicols red of the first order, shows 

 that in all cases the vertical axis is the axis of greatest elasticity, 

 that is to say that the mineral is negative. When both minerals 

 are developed in crystals giving lath shaped sections, the gypsum 

 plate affords a ready means of distinguishing melilite, from 

 nepheline, seeing that in the latter mineral the longer axis of 

 the oblong sections is parallel to the vertical axis. 



In basal sections the " peg structure " appears as little dots. 

 The melilite is often traversed by irregular cracks in addition to 

 the basal cleavage. The accompanying figure (fig. 2) drawn from 

 one of the thin sections of the rock, shows the mineral with 

 better crystalline outlines than it usually possesses, associated 

 with biotite, magnetite and apatite and surrounded by a mass 

 of decomposition products consisting of the most part of 

 calcite. 



