C. Palache — Mineralogical Notes. 



253 



iron, siclerite and hematite the manganese and part of the 

 iron ; quartz may represent silica set free in the reaction. 

 Chlorite is probably derived from biotite, and albite and kaolin 

 from feldspar present in the granite. 



The novelty of this paragenesis for 

 phenacite and the fact that very few 

 occurrences of this mineral are known 

 in New England seemed to make it 

 worth while to record the following 

 facts concerning the mineral contents 

 of this small cavity. 



Phenacite. — Phenacite is in sharply 

 formed yellowish white crystals, short 

 prismatic or lense-shaped in habit, 

 implanted on the quartz of the cavity 

 wall or loose and wholly imbedded in 

 siderite. The crystals vary in size 

 from 5 mm in greatest diameter 

 downward and proved to be suffi- 

 ciently well developed to permit of 

 measurement, although the faces are 

 pitted and somewhat dull. The 

 following forms were _cletermined : 

 m(10_IOJ, a (1120), W1011),_s(0111), 

 j? (1123), o(4223), and a? (1322). The 

 figure* (fig. 2) shows the prevailing- 

 habit, which much resembles that of the phenacite from Flo- 

 rissant, Col., described by Penfield. On many crystals, how- 

 ever, the prism planes are much reduced or entirely wanting, 

 giving a lense-shaped appearance to the crystal. The attach- 

 ment is generally by a side, so that both terminations are seen. 

 The prism planes are brilliant and give good reflections ; the 

 rhombohedrons r and x, which are the dominant terminal 

 forms, are also of fairly good reflecting quality. Faces near 

 the center of the crystal are, however, uneven and appear to 

 have been etched ; the planes of the rhombohedron p are 

 therefore not usually sharp as in the figure but are replaced by 

 a rounded or irregularly facetted surface. As no new forms 

 were observed the measurements are not here recorded. 



Siderite. — Siderite is the most abundant mineral in the 

 cavity. It is transparent with pale brown to pinkish color 

 when freshly fractured, but natural surfaces show an iridescent 

 or steely tarnish. The larger anhedral grains that border the 

 cavity reach a diameter up to 3 cm and show a brilliant cleav- 

 age. Most of the siderite is in the form of cellular aggregates 

 of distinct but rounded crystals of two generations, the older 



* The figure was drawn by Mr. R. W. Richards. 



