56 W. S. Bayley — Rocks of Pigeon Point, Minnesota. 



Associated with the chlorite is oftentimes a very light green 

 fibrous mineral, which from its bright polarization colors is 

 probably to be referred to sericite. Rutile forms quite a 

 prominent constituent in some specimens. It is found in 

 irregular masses of a dark brown color, and also in long rod- 

 like forms, in both cases intermingled with leucoxene and fre- 

 quently with chloiite. 



A second phase of the red rock resembles quartz-porphyry. 

 Well terminated quartz crystals and occasional brick-red and 

 greenish -white feldspars are scattered through a very fine 

 grained groundmass of a dark red or purplish color. This 

 variety is characterized under the microscope by the beauty of 

 its granophyre structure. All gradations between the granular 

 structure just described, and the typical porphyritic structure 

 have been examined, and in all there is more or less of the 

 true granophyre. In the most typical porphyritic varieties the 

 porphyritic crystals are both quartz and feldspar. In the less 

 perfectly developed phases the quartz occurs in round, ellipti- 

 cal and even crescent-shaped areas, and includes in many 

 places portions of the groundmass. This quartz is perfectly 

 clear and is free from inclusions other than the little fluid 

 cavities with movable bubbles (see fig. 1.) 



A very few irregularly outlined feldspar areas represent the 

 porphyritic crystals of this mineral in their earliest stages of 

 development. They are now so altered as to prevent the 

 identification of their species. 



Other areas which appear in the hand specimen as crystals 

 are seen under the microscope to be composed of granophyre 

 substance in which the feldspathic portion is very highly col- 

 ored by little plates of hematite. 



The groundmass, in which these crystals are imbedded, con- 

 sists of quartz and highly altered feldspathic substance in 

 granophyric intergrowths. This is in part sometimes replaced 

 by coarser areas in which the two minerals form a micropeg- 

 matite. The fine granophyre is found more particularly around 

 the distorted and corroded porphyritic quartzes, and in the 

 undeveloped feldspars mentioned above. It radiates from all 

 porphyritic quartzes forming a zone, which in ordinary light 

 resembles the " quartz globulaire " of Levy, but in which the 

 quartz fibers, between crossed nicols, are seen to be optically 

 independent of the orientation of the substance of the crystals. 



Chlorite, iron hydroxides, leucoxene, and tiny fiakes of a 

 dark brown biotite are the accessory constituents of the ground- 

 mass, Calcite is quite abundant' as an alteration product of 

 some of the fibres intergrown with the quartz, and also in the 

 little cavities contained in the rock. Green alteration-products 

 are also common. Fig. 1 is an ideal representation of the most 



