314 PROCEEDINGS OF THE ACADEMY OF [1880. 



Since most of the vermiciilites occur in serpentinous or chloritic 

 formations, it is to be noted that no such rocks occur here or in 

 the vicinit}'. The mineral is probablj' derived orio-iually from 

 hornblende. 



Physical Characters, — Hardness, 1..3 ; Specific gravit}', 2.80 

 (taken in alcohol and referred to water). Lustre pearly. Color, 

 by reflected light, bronze ; by transmitted light, brownish red, and 

 in A'er}^ thin laminae, brownish yellow. Opaque, except in thin 

 pieces. Streak brownish yellow. Laminae unelastic, readily 

 flexible, tough, not brittle. Feel greasy. 



CrystaUographic Characters. — Monoelinic. Cleavage ; basal, emi- 

 nent ; also, occasional!}', a cleavage parallel to the diagonals. 

 Striations crossing at about 90°, caiising the mineral to break 

 into nearly rectangvilar fragments, are sometimes observed, and 

 these are parallel to the plane of the optic axes and to the diagonals 

 of the rhomb. No triangular striations as in Jefferisite. Plates 

 often contorted and wrinkled. Twin crystals frequent, observable 

 by polarized light. Optically biaxial. Double refraction strong, 

 negative. Optic-axial angle, 31°20'-39°30' ; generally 3T°±:. 

 Crj-stals sometimes nearly 2 inches wide and \ inch high. The 

 hyperbolas are well defined in the polariscope, and the angle of 

 their divergence is more constant than in some of the other ver- 

 miculites. Twinning produces variations in the angle.^ 



Pyrognostic Characters. — In the closed tube it gives off water and 

 exfoliates with great force, in a direction perpendicular to its base, 

 to ten times its original volume. Upon exfoliation it becomes of a 

 bright copper color and takes a metallic lustre. It also becomes 

 brittle and more opaque. The exfoliated mineral has a far more 

 distinct and frequent secondary vertical cleavage than it has before 

 exfoliation, and the basal cleavage is also easier. It shows strong 

 double refraction in the polariscope, and has an optical divergence 

 of about the same amount as that of the unignited mineral (30° to 

 37°). The h^'perbolas are extremely ill-defined, and no exact 

 measurements could be taken. It is yellow b}- transmitted light. 

 It forms a fine object under the microscope by reflected light. 

 The fine copper color gained on exfoliation is characteristic, dis- 

 tinguishing it from the other vermiculites. The color is obtained 

 whether it is heated suddenly in the flame, or slowly in an air-bath 

 to exfoliation. Upon long-continued ignition in a platinum cru- 



1 T'. Prof. Cooke's Paper on the Vermiculites, Proc. Amer. Acad., 

 Boston, 1874, 35. 



