884 THE GEOLOGY OP MINNESOTA. 



[Quartz-porphyry . 



No. 2276. QUARTZ-PORPHYRY. (Granitic. ) 



Part of the same rock as No. 2275, but further west. 

 Ref.. Annual Report, xxiv, page 84. 



Meg. Weathering pinkish, approximately a granitic structure, with a few 

 coarse, roundish quartz grains or pebbles. 



Mic. In the main this is a coarser rock than the last, and is more recrystallized. 

 It also contains a larger percentage of free silica. This is in the form of original 

 rounded grains and of minute, interlocking and micro-granulitic secondary quartz, 

 fresh and glassy, some of it being arranged in a vein-like band in which the individ- . 

 ual grains are coarser than throughout the rock generally. There are in the slide 

 none of the original bipyramidal quartzes. The feldspar* are in the form of frag- 

 ments of uniform size. They never interlock, and rarely come into contact. Many 

 of them are plainly triclinic, but quite a number appear to be of orthoclase perhaps 

 one-half of them. The arrangement of a few linear shreds of hornblende and of Hot if c 

 is so generally roughly parallel with itself that there seems to be a trace of an old 

 structure, which, if the hornblende were more abundant, would perhaps develop 

 into a coarse schistosity. The secondary quartz surrounds the fine biotites. While 

 most of the old feldspars are still perfectly evident, there are some that are nearly 

 lost in a fine granulation, caused by decay and subsequent regeneration. Such f eld- 

 spathic grains can be distinguished from the recrystallized matrix by the greater fine- 

 ness of the -structure in the areas occupied by their sections, their greater obscurity 

 or their greater brightness and sometimes by the greater abundance of fine mica 

 scales, or by an identifiable remnant of the original feldspar itself. In the last case 

 there are apt to be a few peripheral secondary fine feldspar grains attached to the 

 old feldspar. 



The isolation of the old feldspar fragments in the regenerated fine matrix is 

 illustrated by figure 5, plate V. That these are fragments and not phenocrysts 

 developed in a magma is evident from the following considerations: 



1. They never show crystal outlines, although they are perfectly unimpeded 

 by the surrounding rock substance. 



2. In size they graduate from the coarsest to the finest, passing into the 

 general matrix. 



3. Their forms are usually subrounded. 



4. This rounding cannot be due to resorption, for the surrounding matrix never 

 enters into them in embayments such as often seen in the quartzes of a magmatic 

 quartz-porphyry. 



5. The only manner in which they are linked with the matrix is by a later 

 regeneration of the rock, by which the matrix has been reformed and the peripheral 

 portion of the old feldspars has been simultaneously developed by secondary growths 

 that interlock in the regenerated matrix. 



