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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 57. 



between crossed nicols, the mineral evidently being approximately 

 uniaxial. The birefringence is higher than that of diabantite where 

 the scales lie upon a face perpendicular to the basal cleavage. The 

 optical character is negative. Occasionally a scale shows hexagonal 

 outline and curved vermicular aggregates occasionally occur. The 

 minimum and maximum indices of refraction are approximately 

 1.560 and 1.575, respectively. The analyses gave the results tabulated 

 below. 



Analyses of stilpnomelane, from Lane's No. IV quarry. 



It was disconcerting to find that in a very large number of occur- 

 rences of green chlorite, the only two suitable for accurate analysis 

 belonged to distinctly different species. All specimens were then 

 reexamined, an attempt being made to determine silica if nothing 

 else. This was in many cases possible and the results seemed to 

 show that but the two species were present, that they did not inter- 

 grade and that individual specimens were distinctly referable to one 

 or the other and none were either mixtures of the two nor intermediate 

 compounds. Quite early in the investigation a reliable qualitative 

 means of distinguishing these two minerals was found in their 

 behavior when boiled with 1.20 specific gravity nitric acid. Powdered 

 diabantite dissolves slowly without change in the acid while stil- 

 pnomelane is wholly oxidized yielding a rust-like brown substance. 

 Through handling many specimens the writer became able to dis- 

 tinguish the two by megascopic features alone. Under the micro- 

 scope they may be readily distinguished by the difference in refrac- 

 tive index and this means served to classify a number of specimens 

 where the amount of the mineral was insufficient for a chemical 

 examination. In one or two specimens anomalous optical properties 

 served to suggest that a third clilorite might be present or that 

 diabantite and stilpnomelane might be intergrown. Most of the 

 greenish-black varnish-like material on joints and slickensides is 

 diabantite as is most of the very fine-grained green chlorite in the 

 altered rock. In the old No. 1 quarry, many small veins and frac- 



