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



Art. XI Y. — Alnoite containing an uncommon Variety of 

 Melilite; by C. H. Smyth, Jr. 



In an earlier paper* the writer described a small dike occur- 

 ring in a fault fissure at Manheim, N. Y. The rock exam- 

 ined was much decomposed, but showed under the microscope 

 abundant olivine, dark brown mica, magnetite, and perofskite, 

 with considerable quantities of alteration products. Upon this 

 mineralogical composition was based a determination of the 

 rock as peridotite. Some sections showed, in addition to 

 the minerals named, another constituent, " of imperfect lath 

 shape," in color white, green or brown, and with a marked 

 fibrous structure perpendicular to the longer axis. The struc- 

 ture in some of the laths resembled the peg structure of meli- 

 lite, and suggested that they might belong to this species ; but 

 their optical properties, notably the yielding of second order 

 interference colors, seemed to preclude the acceptance of such 

 a supposition. 



Recently, however, some sections have been prepared from 

 fresher material, and these show that this first supposition in 

 regard to the nature of the mineral was correct. It is melilite, 

 though in one respect quite different from that mineral as 

 usually described. 



In the fresh material the laths are much more abundant than 

 in the altered rock, and possess a more definite and uniform 

 character. Instead of the fibrous structure they show a very 

 perfect peg structure, while the optical properties, with a 

 single marked exception, agree closely with those generally 

 ascribed to melilite. On the other hand, in the weathered 

 rock from which the first sections were made the melilite is so 

 greatly altered that it gives very imperfect evidence of its 

 original character. These sections afford a good illustration 

 of the rule stated by Stelznerf in regard to other rocks of a 

 similar character: the melilite is often completely decom- 

 posed while the olivine remains quite fresh. Adams,;}; also, 

 has stated that in the alnoite of Ste. Anne, Canada, " the 

 melilite is well seen only in sections of the fresh rock," being 

 no longer easily recognizable " when the rock is decomposed 

 to any considerable extent." 



* A Third Occurrence of Peridotite in Central New York, this Journal, III, 

 xliii, p. 322. 



f Stelzner, A. : Ueber Melilith und Melilithbasalte, Neues Jahrbuch fur Mine- 

 ralogie, etc., Beilage-Band II, p. 388. 



X Adams, F. D. : On a Melilite-bearing Rock (Alnoite) from Ste. Anne de Belle- 

 vue, near Montreal, Canada; this Journal, III, xliii, p. 278. 



