266 THE GEOLOGY OF MINNESOTA. 



[Gabbro. 



instances the twinning lamellae are obliterated. This has extinction on the brachy- 

 pinacoid at 64, indicating 1>i/toir>iifr, m-ar atiort/u/te. Intimately ingrown, in irregular 

 areas in this feldspar, is another which is clear and glassy, and which extinguishes 

 not in unison with the other. These areas ramify singularly, like the spreading 

 growth of a mineral that is produced at the expense of another. In .some cases the 

 secondary feldspar has wholly replaced the original, so far as seen in the section, and 

 the area appears like quartz.* This secondary feldspar is andesine, as indicated by 

 the extinction on n t at 9. In another section, the acute bisectrix being n f , the angle 

 of extinction is 3 on the edge 100:010, indicating andesine-oligoclase. In the centre 

 of the same crystal, which is fresh and set off from the border as from a peripheral 

 zone, extinction is 8, and the acute angle contains n t , indicating again andesine. 

 Thus it appears that this feldspar changes, with the optic angle, from anorthite to 

 andesine and to andesine-oligoclase. 



The pyroxene is diallagc, having a secondary cleavage parallel to 100, charac- 

 teristic of that species. The grains are not altogether thus affected. It was cotempo- 

 rary with or slightly preceded the feldspar (/. e., the original feldspar). There are 

 also later pyroxenes that are better preserved and sustain ophitic relations with the 

 feldspars. 



Three sections. 



Age. Cabotian; Beaver Bay diabase(?) 



Remark. No. 222 differs remarkably from 221, although they are in contact, 

 and must be considered the older rock. According to the field description No. 221 

 is basaltic, further east, and rises into the crests of the hill range which passes inland 

 at Sickle bay. N. H. w. 



No. 223. GABBRO (?) 



Four miles east of the mouth of the Brul^ river. Rises in a bluff facing Sickle bay toward the east, thirty 

 feet high, and having nearly vertical basaltic structure. Evidently the same rock mass as No. 221. (See No. 

 540.) 



Ref. Annual Report, ix, page 57; Bulletin ii, page 101. 



Meg. This rock is much fresher than No. 222. In the sun-light are many 

 reflecting cleavage surfaces of feldspar, but the rock is in general coarse, dark and 

 heavy. 



Mic. The feldspars are not earlier than the pyroxene, but these minerals seem 

 to have formed simultaneously. In the large slide examined not an instance of 

 ophitic structure is discernible, but in rare instances the angite is entirely enveloped 

 by the feldspar. The aitgite has a small optic angle. Considerable amounts of the 

 unindividualized magma were still in the rock when it solidified, but these are now 

 altered to a green and isotropic substance, in the midst of which is rarely seen a little 



*Some such small areas were mistaken for quartz, in his description and illustration, by Dr. Wadsworth. Bulletin ii, 

 plate VI, figure 2, Minnesota Survey. These areas all give a curved black bar in convergent light. 



