C. H. Smyth, Jr. — Alnoite containing Melilite. 105 



In the fresh rock the laths show much variation in size, 

 averaging perhaps about -25 mnl in length, but often reaching 

 •5 mm . Their outline is sometimes irregular, sometimes sharply 

 rectangular. Basal sections of the mineral are not common ; 

 when seen they usually have an imperfect quadratic shape. 

 The most conspicuous feature of the laths is the peg structure, 

 consisting of small peg-like bodies, usually starting from the 

 basal planes, and extending partly or wholly across the lath. 

 This struct ui e is very unevenly distributed, being quite absent 

 in some cases, and in others extending through the entire lath. 

 Again, it may be confined to one-half of a lath, while the 

 other half is quite free. Sometimes the pegs lie in the central 

 portion of the lath, and do not extend to either face ; they 

 may also be elongated parallel to the long axis of the lath. As 

 a rule the outline of the pegs is rather irregular, though it 

 not uncommonly approaches the sharpness and symmetry of 

 Stelzner's figures.* In the course of alteration the peg struc- 

 ture becomes obscured by the development of a fibrous struc- 

 ture, and the outlines of the laths lose sharpness, quickly 

 destroying the characteristic appearance of the mineral. 



Many laths show no trace of cleavage, but in others it mani- 

 fests itself by a single crack parallel to the long axis. Less 

 often two or more cracks appear. In the earlier stages of 

 decomposition the cracks are sometimes accentuated by the 

 formation of alteration products. 



In the thinnest parts of sections the melilite is colorless, 

 elsewhere it has a decided yellowish tint. Compared with 

 melilite from other localities its mean index of refraction is 

 perhaps a little lower, while the double refraction is rather 

 stronger, a yellow interference color being obtained in mode- 

 rately thin sections. But while differing very slightly from 

 other melilites in these respects, it shows a marked divergence 

 in another property. Examination of the laths with a gypsum 

 plate sho^s that their long axis is the direction of greatest elas- 

 ticity, and the short axis the direction of least elasticity. As 

 the mineral is tabular parallel to the basal pinacoid, this gives 

 for its optical orientation a — a and v — c, co <e and the mineral 

 is positi ve. While this is true of a large proportion of the 

 melilite, it is not invariably the case, for laths are occasionally 

 found in which the mineral is negative. More common is the 

 occurrence of laths, which, though consisting chiefly of posi- 

 tive material, have patches of a negative character scattered 

 through them. 



The negative character of melilite as a rock constituent has 

 been so generally regarded as constant that any exception to 



* Op. cit, Taf. VIII, fig. 4. 



