No. 2316. MINERALS FROM WESTFIELD, MASS.— SHANNON. 403 



varies slightly in different specimens. The minute scales vary in 

 color and transparency and appear as though dusted with a sub- 

 microscopic brown pigment, quite probably limonite, which has 

 separated out in the process of oxidation. 



In a recent paper Emerson * gives a brief description of specimens 

 from this locality, in which he mentions crystal cavities or molds of 

 anhydrite now entirely removed. In some cases the anhydrite has 

 been replaced, he writes, by diabantite, which has altered to diaban- 

 tite-vermiculite and then to limonite. Many specimens of these flat 

 negative crystals are included in the writer's collections from these 

 quarries. The hollow cavities are of several forms which are not aU 

 of the same age. So far as can be determined from these specimens 

 the chloritic mineral is all stilpnomelane and the golden product of 

 its alteration, as above mentioned, is chalcodite and not a vermicu- 

 lite. The complete change to limonite was not observed, although 

 when very fine grained the chalcodite might readily be mistaken for 

 limonite. Judging from the fact that the interior surfaces of the 

 stilpnomelane pseudomorphs are always clear and sharp where the 

 anhydrite or other mineral has been removed with no rounding of 

 angles, etc., and the exterior of the stilpnomelane crust is indistinct 

 and irregular the writer has regarded the stilpnomelane as having 

 encrusted the crystals of the reraoved mineral without replacement. 

 The penetration along cleavage planes where it occurs is in laminae so 

 sharp and uniform in thickness as to suggest simple filling of cleavage 

 cracks opened by strains. The crystal cavities and molds will be 

 described more fuUy in a later paper on the paragenesis and miscella- 

 neous minerals which with a paper on the crystallography of the 

 datolite wiU conclude this series. 



It may be well here to state the nomenclature used in designating 

 the quarries. Going north from the Boston and Albany Railroad these 

 are numbered 1-la, 2-2a, 3-3a, and 4 respectively, this being sub- 

 stantially the designation used by the quarrymen who include the 

 two openings each of the first, second, and third quarries under the 

 single number. 



The type material described above has been deposited in the 

 United States National Museum, catalogue numbers as follows: 

 Diabantite 93440, 93441; stilpnomelane 93447, 93438; chalcodite 

 93442. 



J Emerson, Araer. Journ. Sci., vol. 42, 1916, p. 233. 



