00 THE GEOLOGY OP MINNESOTA. 



[Granite. 



minerals are present, in the order of abundance: feldspar, quartz, hornblende, a greenish 

 yellow, almost isotropic, mineral, may net iff, epidote, muscovite and apatite. 



The feldspar is much altered and reddened, as is usual in rocks of this class, and 

 has sometimes become opaque; thus some of the areas that were originally feldspar 

 show almost no action on polarized light, and many of them have comparatively 

 little of the feldspar material in its original state. However, in a few cases, tho 

 traces of polysynthetic twinning can be noticed, but not frequent nor distinct enough 

 for determining the nature of the feldspar. Several simple twins, apparently of 

 orthoclase according to the Carlsbad law, are seen, and also many untwinned grains, 

 some of which are less altered than those which show twinning. From these facts, 

 and from the analysis which is given below, it appears that the feldspars present are 

 (1) Orthoclase, in some quantity; (2) Plagioclase of an undetermined variety, but 

 probably near oligoclase; and (3) Anorthoclase is supposed to be present, as it is 

 known to occur quite frequently in rocks of this class. 



As secondary minerals in the feldspar are muscovite, quartz, epidote and a 

 greenish yellow mineral. The last has almost no effect on polarized light, occurs in 

 irregular areas or in branching vein-like forms, and is intimately associated with a 

 fibrous mineral of about the same color; the latter appears to be hornblende. Epidote 

 is seen in small areas and crystals; it is yellowish or colorless. Muscovite exists in 

 small flakes. In altered feldspar crystals three zones can sometimes be distinguished ; 

 an inner one composed of rather fresh feldspar, outside of which is an opaque reddish 

 zone more altered than the interior, and beyond this a zone of about the same 

 nature, but less reddened. Very frequently the inner zone, instead of being the less 

 altered, has almost no feldspar material left, but is composed largely of the isotropic 

 mineral mentioned above, oftentimes associated with finely fibrous hornblende, 

 epidote and a few muscovite flakes. 



After the feldspars the most important ingredient of the rock is qinniz, which 

 occurs in two modifications. The first form occurs in the spaces between the feld- 

 spars. It has all the characters of ordinary granitic quartz. It is clearly younger 

 than the feldspar by which its outlines are conditioned. The other form taken by 

 quartz is an intimate micropegmatyte with the feldspar. This micropegmatyte per- 

 vades the whole rock and is a very characteristic microscopic feature of the granular 

 acid rocks of the Cabotian. As the feldspar is so much altered and darkened, this 

 structure is very easily seen in ordinary light. Large areas of feldspar are sometimes 

 completely penetrated by this network of quartz; again, the quartz particles occur 

 only around the peripheries of a feldspar grain. These apparently detached quartz 

 grains are sometimes oriented with the larger quartz grains adjoining the micropeg- 

 matyte, and sometimes are entirely independent of them. This rock does not show 



