168 THE GEOLOGY OF MINNESOTA. 



[Quartz. Diabase. Tlialitc. 



zeolite can be so directly referred to the alteration of the feldspar. Thomsonite is 

 a frequent mineral filling amygdaloidal cavities, about Grand Marais, but rarely 

 gathers in the cleavages of the feldspar from which it seems to be derived. Mr. A. 

 H. Elftman, during 1895, while at work for this survey, found in the anorthosyte at 

 Carlton peak and north of Beaver bay small masses of a mineral like thomsonite 

 resulting from an alteration of the labradorite of the rock. This proved to be 

 mesolite. (American Geologist, xxii, 30.) r. s. G. 



No. 91. DIABASE. 



East side of Knife river, extending for a quarter of a mile, passing under the rock of the point, No. 90; with 

 amygdulesof white minerals. (Compare No. 641.) 

 Kef. Annual Report, ix, pages 24, 26. 



Meg. Medium-grained, dark diabasic rock, considerably altered, having in its 

 openings a saponite-like mineral in masses sometimes several inches across. 



Mic. From the poor section at hand it can be observed that the rock is an 

 ophitic diabase, the pyroxene in large crystals, but considerably clouded by granular 

 opaque impurities. Whether it ever contained olivine it is impossible to discover 

 from this section. Another section shows much glassy remnant. 



One section. 



Aye. Cabotian. N. H. w. 



No. 91A. QUARTZ. 



From cavities in No. 91. 



Ref. Annual Report, ix, page 25. 



This quartz is milky or cloudy white, amorphous or irregularly granular, and 

 contains masses of the next, both small and large. 



No section. N. H. \v. 



No. 91B. THALITE (w&h quartz, calcite, etc). 



From cavities in No. 91. 



Ref. Annual Report, ix, page 25. American Geologist, vol. xxiii, page 41, January, 1899. 



Meg. The larger masses, which are cream colored and amorphous or massive, 

 are of composite nature^ embracing not only thalite, but more or less of calcite, 

 quartz and laumontite, outwardly resembling kaolin. 



The mineral composing the smaller and softer nodules was probably included 

 in what was named thalite by Owen.* It is apparently amorphous, but in thin 

 section it is seen to be finely fibrous. 



Mic. It has high double refraction, positive elongation, and parallel extinction, 

 forming spherulitic and vermicular shapes, sometimes affording a constant black 

 cross. It is not perfectly transparent, but has a dull translucency. In the centre of 



* Geological Report on Iowa, Wisconsin and Minnemtu, p. 600. 



