234 THE GEOLOGY OF MINNESOTA. 



[Thomsonite. Diabase. 



finer mineral, the pyramids of one fitting into the intra-pyramidal spaces of the 

 other. This is not an evident structure, but, along with a close and somewhat con- 

 fused fibrosity, the fractured surfaces sometimes reflect light in such a manner as to 

 bring out such an oblique interlocking. Yet, notwithstanding this apparent differ- 

 ence between the centres and the peripheries of these masses, there are visible flat 

 areas where no such structural contrast appears, and the two grade together with 

 the most gradual and invisible variation. Hence it is necessary to conclude that 

 there is really but one mineral. 



Mic. Thin sections were made from the coarsely radiated portion, one perpen- 

 dicular and one parallel to the macro-structure, from which it is quickly observed 

 that the axial plane is perpendicular to the structure, and that n.. is perpendicular to 

 the main cleavage. The elongation, hence, is sometimes positive and sometimes 

 negative, n g being the acute bisectrix. The section cut perpendicular to 6 gives no 

 polarization colors, or very faint ones, but all others are colored, even in very thin 

 sections. The section cut perpendicular to the structure is parallel to ?*,, and 

 gives the highest coloration. This section also proves, by the evident parallel lining, 

 that the structure, at least in the coarser portions, is lamellar, rather than fibrous. 

 It is evident, therefore, that toward the centres of the white masses, where they begin 

 to show red and pink colors, the lamellar structure gives place more and more to a 

 minute and densely fibrous structure. Extinction is parallel to elongation. 



This mineral fuses readily to a porcellanous bead, and swells. With HC1, it gives 

 flocculent silica which slowly gelatinizes. 



With the Boricky test with hydrofluosilicic acid, the crystalliths that appeal- 

 indicate lime and soda. 



It is therefore thomsonite. 



Three sections. 



Age. Manitou. . N. H. w. 



No. 164. DIABASE. 



From one of the much altered traps, alternating with amygdaloid, similar to the series of Agate hay, 

 between Sugar Loaf point and Two Island river, dipping toward the lake at an angle of about 12; permeated 

 with saponite. 



Sef. Annual Report, ix, pages 43, 44. 



Remark. The section numbered 164 is evidently not from this rock, but is from 

 a rather fresh olivine and ophitic diabase, probably No. 165. 



Age. Manitou. N. H. w. 



No. 165. DIABASE (vnth olivine) . 



From the islands of Two Island river. The rock rises perpendicular from the water on the westerly side 

 of these islands, basaltic, dipping southeast conformably with the rocks of the coast. 

 Ref. Annual Report, ix, pages 43, 44. 



