M ELI LITE - NEPHELIAE - BA SAL IS. 343 



to be nearly equal, 20°-22°. According to this measurement the 

 prism must have approximately the position of (940); the angle 

 of the latter upon ( 1 10) is 21° 2', the angle upon (lOo) =23° 58'. 

 A particular phenomenon in the growth of the melilite is the fact 

 that the base does not generally present an even plane, but shows 

 a conical depression. The shape of the lath--shaped sections 

 then resembles the profile of a biconcave lens. Sections parallel 

 to the base are isotropic between crossed nicols and show, when 

 they are nat too thin, an indistinct dark cross in convergent light. 

 The cleavage parallel to (001), the cross-fibration of the lath- 

 shaped sections and the occurrence of the spear-shaped and peg- 

 shaped inclusions arranged parallel to the c axis (the so-called 

 Pflockstniktur) are very distinct. Inclusions of pyroxene, mag- 

 netite and glass are common ; as already mentioned, these inclu- 

 sions are generally arranged in zones. In sections parallel to 

 (001) they fill the central parts of their host, and often make 

 up two or three concentric zones. These sections closely re- 

 semble leucite because of their rounded shape, the arrangement 

 of the inclusions and the lack of double refraction. Melilite 

 becomes nearly colorless and transparent, but in comparing it 

 with the white, colorless nepheline, it shows a feeble yellow tint. 

 Decomposition has taken place to only a small extent; it be- 

 gins along the cross-fibration, and greenish-yellow alteration- 

 products result, the fibres of which are perpendicular to the length 

 of the lath-shaped sections. 



Nepheline is always fresh, colorless and transparent ; it rarely 

 exhibits a regular shape, but generally forms an aggregate of 

 irregular grains, cementing the other components ; it is evidently 

 the latest formed mineral in the rock. 



There is abundant magnetite besides perovskite, the common 

 associate of melilite, which occurs in small octahedrons and ir- 

 regular grains. The perovskite becomes transparent with a 

 brownish-violet color, and shows in some sections a feeble, ab- 

 normal double refraction. There appears to be no isotropic base 

 in the normal rock, but if any is present, it must be in a very 

 small amount. There are coarser grained spots in the rock, 



