716 THE GEOLOGY OF MINNESOTA. 



[Esterellyte. 



section shows, at the same time, no albite or other twinning. It is hence about 

 parallel to 010. A section showing n f nearly perpendicular has both albite and 

 pericline(?) bands. The extinction angle here, between the optic plane and the 

 albite bands, is 72 to 74. The angle of the axes round n v is evidently larger 

 than round n f . The angle between the albite and pericline(?) macles on a section 

 cut perpendicular to n v is 87 30 '. It is noticeable that the pericline(?) bands 

 extinguish simultaneously with the albite bands, set by set, as in microcline twinning, 

 although the albite lines are long and frequent, and the periclines(?) are short, far 

 apart and infrequent. The twinning of all the crystals is coarse, very abun- 

 dant and distinct. There are also twinnings on the Carlsbad and Boveno plans. 

 Some of these characters indicate labradorite or andesine, and some indicate 

 anorthoclase. 



The pyroxene is also interesting. It is, in general, green, but frequently the 

 color fades out in patches, or at the centre, which extinguishes at a different angle. 

 Indeed, there is an irregular zonal structure in some of the crystals, this being made 

 evident by the successive extinctions, there being four, and indistinctly sometimes 

 six different extinctions in a crystal favorably cut. These are distributed in zones 

 which are generally better developed at one end of the crystal than at the other. 



As shown by the diagram above, the axis n g is in the acute angle of the crystal, 

 making a large angle with the vertical axis, which is an anomaly for augite, indi- 

 cating the soda-bearing aegyrine. There seems to be no exception to this in the 

 numerous sections (010) of the slide. 



There is a little biotite and some scattered grains of sphene in the rock. One 

 section. 



Age. Archean (intrusive in the Upper Keewatin). 



Remark, See under No. 1094. N. H. w. 



No. 1400. ESTERELLYTE. 



From the south side of Kekequabic lake, near the groat diabase dike which crosses the lake, making a 

 couple of islands, 



Ref. Annual Report, xvi, pages 101, 102, 124. ' 



Mey. Similar in aspect to Nos. 1398 and 1399. Compare No. 1061. 



Mic. It is unlike the last in having lost its augites, which have become 

 altered to actinolite, some of which is blue in common light. The rock has 

 also taken in a notable amount of calcite. Sphene is quite common. One 

 section. 



Age. Archean (Keewatin). 



Remark. It is noteworthy that the effect of the dike with its hot water and 

 gases has been to destroy the augites in the porphyry. N. H. w. 



