143 ANWALS XEW YORK ACADEMY OF i^CIEyCES 



tached portion of the section shows calcite and quartz, with a shred of 

 amphibole. Using the birefringence of the quartz as a basis, the amphi- 

 bole gives a maximum birefringence of 0.025, which agrees very well with 

 arfvedsonite. The possibility of segirite was considered, but the proper- 

 ties agree much less perfectly with those of that mineral. ]S^ot many 

 occurrences of arfvedsonite in this country have been noted and these 

 only in eruptive rocks. A further study of its properties with more cer- 

 tain identification would be of interest. It is to be regretted that 'so little 

 of it has been found. In slide 153, containing pectolite and large albite 

 crystals, a few shreds appear to be the last remnants of arfvedsonite. In 

 108, a single shred shows the characteristic pleochroism (blue-green and 

 lilac) and in 77, a half dozen remnants show pleochroism as in 116, — X 

 blue-green, Y claret and Z yellow-green, with X > Y > Z. A search has 

 been made through a large number of hand specimens in the endeavor to 

 find other occurrences. A few cases were observed, but always so much 

 decomposed that thin sections could not be obtained. In its typical habit, 

 the mineral consists of groups of tabular crystals or somewhat scattered 

 individuals resting upon or mtergrown with quartz, datolite, pectolite or 

 albite. It was always decomposed to a large degree to a grayish-green, 

 pulverulent substance, or its total removal had left narrow gashes in the 

 associated minerals. An interesting feature brought out was that similar 

 gashes were not uncommon in many specimens in which there was no 

 other evidence of arfvedsonite. Their frequent occurrence suggests that 

 arfvedsonite may have been formed in considerable quantity at one stage 

 of alteration but that in most cases it has totally disappeared. 



Varieties 2 and 3. The usual amphibole found in the slides is of very 

 different character. It occurs in tufts or bundles of fibres or in groups of 

 slender prisms, and in general character resembles actinolite. It is found 

 in small quantities in a great number of the thin sections, but in several 

 in which the minerals associated with it are those belonging to the first 

 periods of alteration, it is present in notable amounts. In 98, it occurs in 

 tufts of small fibres, dispersed in columnar or plumelike forms, and at 

 times occupying most of the field (Plate XIII, fig. 1). Pleochroism 

 strong, from a clear emerald-green parallel to Z to pale brown tinged with 

 green at right angles to Z. Xo difference in pleochroism l)etween X and 

 Y can be perceived. Absorption strong parallel to Z, weak parallel to X 

 and Y. Extinction nearly parallel to elongation of the needles, but angles 

 up to 11° were obtained. Direction of elongation makes an acute angle 

 with the Z-axis of elasticity. Index of refraction greater than prehnite 

 and approximating that of calcite, indicating a value of about 1.G5. The 

 maximum interference color in the section is a blue of the first order, 



