476 THE GEOLOGY OP MINNESOTA. 



[Apotrachyte. 



Mic. Porphyritic feldspars, much altered and reddened, probably orthodase, 

 are seen in a groundmass which consists of reddened feldspar, quartz, hematite, indis- 

 tinct greenish-yellow alteration products, chlorite, magnetite and a small amount of 

 epidote. The most noticeable feature of the groundmass is the presence of many 

 spherulitic areas, which are usually more apparent in ordinary than in polarized 

 light. The spheruliths have a distinct concentric, as well as the radiating, structure. 

 The different zones are not clearly developed in each individual, but the most perfect 

 ones show the following zones, beginning with the centre: (1) A nearly opaque 

 mass showing an indistinct radiating structure. Under crossed nicols this structure 

 disappears, and sometimes the mass breaks up into a tine mosaic, probably of quartz 

 and feldspar. (2) A more transparent zone, without radiating structure, and showing 

 under crossed nicols a very fine-grained mosaic of probably quartz and feldspar. 

 (3) An opaque greenish-yellow band. (4) A more transparent zone, similar to the 

 second zone just mentioned. (5) A broad zone, commonly the largest part of the 

 spherulith, somewhat opaque and of a red color. The radiating structure is very 

 pronounced, and under crossed nicols the less reddened spheruliths give in this zone 

 the arms of a black cross. The fibres of this radiating zone show approximately 

 parallel extinction and a negative elongation; they can thus be orthoclase elongated 

 in the direction of the inclined axis (a). 



It is quite frequently the case that the porphyritic feldspars are surrounded by 

 spherulitic growths in which the central nearly opaque zone mentioned above is 

 immediately in contact with the feldspars. 



Aside from the spheruliths, which make up a large part of the rock, the ground- 

 mass consists of the minerals already mentioned. In places a fine mosaic of quartz 

 and feldspar is evident, most likely a product of devitrification, and in other places 

 larger quartz grains occur; these commonly show undulatory extinction and some 

 of them exhibit a tendency to a radial structure. There are some altered areas of 

 the groundmass which appear to have been originally biotite. 



Two sections examined. 



Age. Cabotian. 



Remark. This is the first specimen known to us of the red rocks in Minnesota 

 containing spheruliths. With such a large number of occurrences of acid surface 

 rocks along and near the north shore of lake Superior, it is a little surprising that 

 spherulitic structures have not been found. It is to be expected that future search 

 will reveal such occurrences. (Compare Nos. 686 and 687.) 



In rock No. 132A a spherulitic structure is described in red rock inclusions in a 

 basic eruptive. u. s. G. 



