420 



THE GEOLOGY OP MINNESOTA. 



[Thomsonite. Epidote, calcite. Diabase. 



No. 558A. THOMSONITE. 



From cavities in No. 558. 



Ref. Annual Report, x, page 50. 



Meg. This mineral is light lilac gray, or a pinkish white or white, with a 

 hardness 5 or 6. Its structure is massive, or fibrous and divergent, the rays 

 becoming separated, acicular orthorhombic crystals. Some of the masses are two 

 inches, or more, in diameter. 



Elongation t- tung atiur, 



FIG. 24. ORIENTATION OF THE FIBRES OF THOMSONITE. 



Mic. The structure is divergently fibrous, the fibres having both positive and 

 negative elongation and a parallel extinction. The axial plane is perpendicular to 

 the fibres. In a section of a lot of fibres showing vertical, the axial angle is so 

 small that the hyperbolas do not leave the field of the microscope. The double 

 refraction is rather high, the fibres that are cut obliquely to n m giving colors of blue 

 and even green. The highest color (for the thickness of the section, which is about 

 0.03 millimeter) would be the second yellow, found in fibres cut perpendicular to 

 their elongation. But few if any such appear in the slide. The fibres are coarse, 

 compared to the zeolites which have passed as thomsonite for several years, derived 

 from the vicinity of Grand Marais. They are not straight and rigid, but somewhat 

 alternating and fluxuous, with a feathery structure often prevalent. Figures above 

 show the position of the axes of elasticity and the cleavage. 



One section. 



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



No. 558B. EPIDOTE AND CALCITE. 



Veins in No. 558. 



Kef. Annual Report, x, page 51. 



Iii this green epiitote is a little metallic copper. The 



No section. 



Aye. Cabotian(?) 



No. 559. DIABASE (with olivine). 



Immediately overlies the cupriferous conglomerate of the Island mine. 



Meg. A much-rotted, gray, medium-grained rock. 



iic is in scattered nests. 



N. H. w. 



