H. S. Washington — Solvsbergite and Tinguaite. 183 



crysts of labradorite, some of which measure over 10 cm in 

 length. The tinguaite was discovered by Mr. J. H. Sears of 

 Salem, Mass., who described it in 1893.* It is also briefly 

 noticed by Rosenbusch,f who speaks of it as " an eminently 

 fresh and typical tinguaite, with rather greasy -luster, and 

 lighter in color than usual. The small, rare phenocrysts are 

 microcline and microcline-microperthite. In the groundmass 

 there appears to be present some analcite pseudomorphous 

 after leucite." 



Although the rock has been described by Sears, yet a descrip- 

 tion of the specimens in my possession will not be out of place, 

 in view of certain features to be discussed later. 



The rock is very compact, tough and aphanitic, with a silky 

 sheen, and the slightly greasy luster common to the tinguaites. 

 Its color is olive-green, varying somewhat in different speci- 

 mens, from greenish gray to. greenish black. A few white 

 feldspar phenocrysts are visible. The specific gravity was 

 determined by the balance to be 2 '474 at 22° C. 



In thin section the freshest specimens (obtained by Mr. Sears 

 from blasting, and toward the center of the dike), sh'ow a 

 structure characteristic of such rocks — needles and larger grains 

 of aegirine, with many needles and a few phenocrysts of feld- 

 spar, lie in a clear, colorless groundmass, which in general is 

 isotropic, but which here and there shows faint double refrac- 

 tion. The rock is perfectly fresh, no trace of decomposition 

 being observed in any of the constituents. There are no evi- 

 dences of flow structure. 



The aegirite occurs as slender needles and as larger, generally 

 elongated, crystal fragments. It is perfectly fresh and carries 

 no inclusions, beyond an occasional minute speck. The ex- 

 tinction angle, measured on many sections which (as shown by 

 the pleochroism) were approximately parallel to b (010), was 

 found to be about 7°. The axis of elasticity lying nearest to 

 c was determined by means of the mica plate to be a. Pleo- 

 chroism is rather feeble ; a slightly bluish green, 6 yellowish 

 green, c slightly brownish green to yellow. The difference of 

 absorption was slight; c about equal to a, and this slightly 

 greater than b. Some of the larger crystals show cores of a 

 brownish yellow, non-pleochroic pyroxene, but this is present 

 in very small amount. 



Only a few of the feldspar phenocrysts were visible in the 

 sections. I could find no microcline twinning, and, as far as 

 my observations go, judging chiefly from the moire appear- 

 ance,^: they seem to be anorthoclase rather than microcline. 



* J. H. Sears, Bull. Essex Institute, xxv. 4, 1893. 

 f Rosenbusch. Mikr. Phjs., ii, 483, 1896. 

 X Cf. Pirsson, this Jourual, ii, 196, 1896. 



