PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 423 



Thomsonite. Calcite, quartz. 



Chlorastrolite.] 



are a lot of secondary minerals calcite, chlorite, hematite (?) quartz, epidote. Two 

 (poor) sections. 



Age. Manitou. N. H. w. 



No. 564. THOMSONITE. 



Zeolitic white mineral from the trap at Chippewa harbor. 

 Ref. Annual Report, x, page 52. 



Me;/. The structure is radiated and lamello-flbrous, rather coarse. 



Mir. The thickness of the only section made is too great to permit of positive 

 determination of the optic characters, being colored, in general, between crossed 

 nicols, in the tints of the third or fourth order. About the edges, however, can be 

 seen small fringes and patches of the brighter tints of the second order, indicating a 

 high double refraction. This, taken with the outward resemblance to No. 558A, 

 may be sufficient to indicate thomsonite. 



One (poor) section. 



Age. Manitou. N. H. w. 



No. 565. CALCITE, QUARTZ, ETC. 



From the beach, Chippewa harbor, Isle Royale. 

 Ref. Annual Report, x, page 52. 



Meg. A piece from the trap composed chiefly of calcite and quartz, with a little 

 of the rock matter attached, and embracing a radiated (coarse) green mineral whose 

 hardness is about 3. 



No section. 



Age. Manitou (?) N. H. w. 



No. 566. CHLORASTROLITE. 



Prom an amygdaloidal bed of trap that disintegrates near the narrows of Chippewa harbor. 

 Ref. Annual Report, x, page 52; American Geologist, vol. xxiii, page 116. 



Meg. The amygdules are, in size, up to half an inch in diameter, but the larger 

 ones are composed of several radiating nests, appearing like chlorastrolite. The 

 fibres are firm, and not easily broken, green in mass, but light-green in single. They 

 are quite short, rigid and uniform, and also fine finer than those of thomsonite, 

 but about the same in coarseness as the fibres of mesolite. (Compare No. 570.) 



Mic. The elongated fine fibres are not brilliantly polarized between crossed 

 uicols. In the space of their elongation from the centre of a spherulith to its 

 circumference, these fibres sometimes undergo variations of color. For instance, at 

 the centre the tint is neutral (or bluish neutral); at a short distance from the centre 

 the same rays, or others that have replaced them, are light yellow, still further the 

 field is wholly of the neutral tint, same as at the centre; then a light-yellow super- 

 venes, and at last the neutral tint returns, making three recurrences of the same 



