180 



THE GEOLOGY OF MINNESOTA. 



No. 112A. CALCITE AND STILBITE. 



[Oalcite and stilbite. Labradorite. 



From a vein in No. 112. 



Ref. Annual Report, ix, pages 29, 30. 



The specimen shows the full width of the vein, having a selvage of rock on each 

 edge, and exhibiting a thickness (in the vein) of five and one-half inches. Of this 

 thickness four and one-half inches consist of calcite, and about one-half inch of 

 stilbite encloses the calcite on each side. The stilbite is orange yellow. Cleavage 

 pieces give the interference figure of ,, the axial plane lying in the plane (010) 

 parallel with the easy cleavage.* N. H. w. 



No. 113. LABRADORITE. (Rock.) 



Embraced in the dark trap at Splitrock point. 



Kef. Annual Report, ix, pages 29, 30; Annual Report, x, pages 40, 139; American Association for the 

 Advancement of Science, vol. xxx, page 102. 



Compare Nos. lloA, 123A, 128, 810, 814, 810, 818. 



Meg. A massive, homogeneous, gray, coarsely crystalline rock, consisting wholly 

 of one mineral, which is a striated feldspar. Specific gravity by Westphal balance, 

 in methyl iodide, is 2.703. 



Mic. A coarsely crystalline plagioclase showing much albite twinning and occa- 

 sional dashes of pericline. Fresh, affording a fine preparation for the microscope, it 

 is one of the most useful rocks of the state for the study of the plagioclases. In 

 thick section it polarizes in brilliant tints, but in those that do not exceed 0.03 milli- 

 meters in thickness, it is in the gray colors of the first order of Newton's scale 

 While the grains are sometimes much shattered by reason of pressure, yet in all 

 directions it affords numerous large plages amongst which can be found those per- 

 pendicular to the "bisectrices and to the optic axes. 



FIG. U. THE FELDSPAR IN ANORTHOSYTE, NO. 113. 



In the figure (No. 6) the symbol )-< indicates that the grain in which it occurs is cut perpendicular to a bisectrix ; O signi- 

 fies an oblique section; g and |i signify sections perpendicular to the bisectrices of least and greatest elasticity respectively, 

 i. e., to the axes expressed by the German symbols c and a. The figures denote the angles of extinction on the grains on which 

 they are written. 







Fine dust-like impurities are seen to intercept the passage of non-polarized 

 light. These are in the meshes of the broken grains, and also run in lines across the 



*Bull. Societe Miniralogie de France, vol. viii (1885), p. 345. 



