432 THE GEOLOGY OP MINNESOTA. 



[Diabase. 



section shows oblique, but these fibres exhibit an irregular and undulatory 

 extinction, and their elongation is sometimes positive and sometimes negative. In 

 the same fibre a small change (in the direction of elongation) in the point of obser- 

 vation will sometimes alter the interference figure from an optic axis to an axis of 

 elasticity. These fibres are nearly always negative in elongation, but sometimes the 

 same individual fibre changes from positive to negative at a cleavage or fissure which 

 crosses it nearly at right angles, or quite obliquely. This indicates the optic plane 

 is transverse to the fibration, as in thomsonite; yet a nearly square trans verse section 

 of one of the fibres shows an optic axis and is less doubly refractive than some of the 

 fibres cut parallel to their length. This may be scolescite, although the maximum 

 angle of extinction is much too large. 



These anomalous fibres cannot be distinguished, in point of view of absorption, 

 from the coarse fibres of thomsonite. They seem to be thomsonite in all points 

 of view except in the wandering positions of the optic plane, which is sometimes 

 even longitudinal. 



One section. 



Age. Cabotian(?) 



Remark. This intimate mixture of two fibrous minerals shows how futile would 

 be a chemical analysis for determination of the species. Many of these pebbles 

 appear on the beach; the survey collection, already depleted by donation and 

 exchange, still contains about seventy-five specimens. 



A Boricky test gave, along with abundant crystals of fluosilicate of lime a few 

 small rods of fluosilicate of soda. When powdered these minerals together do not 

 gelatinize in HC1. It appears probable that thomsonite prevails over prehnite in 

 these pebbles. N. H. w. 







No. 574. DIABASE. 



Extremity of Blake's point, east end of Isle Royale. 

 Ref. Annual Report, x, page 53. 



Meg. A medium-grained diabase, apparently considerably decayed. What was 

 evidently augite originally, now seems to be chlorite, and the feldspars are pinkish 

 and greenish gray in color. 



Mic. The section is too thick for careful study. It shows that the rock is a 

 decayed diabase. The feldspars are cloudy and almost opaque, much radiating 

 chlorite has been developed in the rock, and most of the augite has disappeared. 

 Considerable iron ore (ilmenite or magnetite) is present, and there are stains of 

 hematite. Possibly some olivine was originally present. 



One section. 



Age. Cabotian(?) u. s. G. 



