464 THE GEOLOGY OP MINNESOTA. 



I Labrador! te. Thomsonite and im-sotype. 



There is also a small amount of accessory epidote, and apparently of biotite. 



Two sections. 



Age. Cabotian. 



Remark. This rock might be called pegmatyte, except for the fact that is in a 

 massive bulk instead of being in form of a vein. It was probably formed by solution 

 rather than fusion. It may represent a transformed basic rock. N. H. w. 



No. 037. LABRADORITE. ( AnortJtosyte. ) 



From a large detached mass, one of several at the west point of Beaver bay (see No. 120). 

 Ref. Annual Report, ix, page 32; Annual Report, x, pages 64, 140; Bulletin vi, plate XV; Final Report, 

 vol. i, pages 196-199; American Association for the Advancement of Science, vol. xxx, page 163.) 



Meg. Gray, coarse, fresh, glittering with cleavage surfaces, one of which is an 

 inch in length. Consists apparently wholly of feldspar. 



Mic. The section shows nothing but labradorite feldspar, except as accessories, 

 along a fissure, some ferruginous accumulations and calcite. 



It is hardly necessary to repeat observations on this feldspar. The plate at 

 hand is large and presents a beautiful section of this beautiful rock. However, in a 

 slide, 010 gives extinctions in three different crystals, respectively 20, 24 and 31. 

 There are no good sections, n,, or?\, but in one instance an oblique section on ogives 

 extinction at 54, and one on n p , at 55 30', both indicating anorthite. Still, the 

 preponderance of evidence is in favor of a labradorite, as before. 



Three sections. 



Age. Cabotiau. N. H. w. 



No. 637A. THOMSONYTE AND MESOTYPE. 



From seams in No. 637. This mineral comes off the joint surfaces of the rock No. 637 as a scale about a 

 quarter of an inch thick, but sometimes half an inch. 



Ref. Annual Report, x, page 64; Annual Report, xi, pages 172, 181. 



- Meg. Light flesh-colored or white, finely fibrous divergently, with numerous 

 points from which the fibres radiate, situated in the walls of the fissure. Thus the 

 growths from opposite sides of the fissure meet along the centre of the seam. Scat- 

 tered calcite crystals were formed on the walls of the fissure before the zeolite was 

 deposited, and these were then buried under the zeolite. On weathering out they 

 leave their hexagonal impress as molds in the compact and hard zeolite. 



Mic. This mineral is both positive and negative in elongation, and has rather 

 high double refraction and parallel extinction. It is plainly quite pure. A micro- 

 chemical test showed lime and soda. The zeolite may therefore be pronounced thom- 

 sonite, which is a prevalent mineral in situations where labradorite alone seems to 

 have contributed to its formation. 



There is another section, numbered "637A, across the vein," which appears 

 different, and it is probable that one or the other is misnumbered. This mineral 



