W. S. Bayley — Rocks of Pigeon Point, Minnesota. 55 



(II.) Macroscopical and Microscopical. 



The red rock of Pigeon Point presents several phases differ- 

 ing in some respects from one another. In its most typical 

 aspect it is a brick-red, fine grained, drusj rock, speckled with 

 little spots of a dark green color. Scattered through the pre- 

 vailing red feldspar are small grains of white quartz, which 

 sometimes present well-defined crystal outlines. The feldspar 

 itself is occasionally observed with a well marked cleavage 

 and rarely with a crystal outline. Usually it has no distinctive 

 morphological characteristics. In some cases a smallquantity 

 of alight colored feldspar can be detected intermingled with the 

 red variety. The green spots consist of little plates of chlori- 

 tized mica. 



Under the microscope the coarser grained of these non- 

 porphyritic varieties are seen to be composed essentially of an 

 hypidiom orphic granular aggregate of at least two feldspars, 

 quartz and chlorite, with a few subordinate constituents — mus- 

 covite, rutile, leucoxene, magnetite, hematite and apatite. 



The feldspars embrace a striated plagioclase, twinned accord- 

 ing to the Carlsbad law, and in one instance according to the 

 Manebach law, and a second, less well individualized feld- 

 spar, which is younger than the plagioclase, but slightly older 

 than the accompanying quartz. It surrounds the plagioclase 

 and is intergrown with the quartz in micro-pegmatitic and 

 granophyric forms. Both the plagioclase and the granophyre 

 feldspar are colored by numerous little plates of hematite, the 

 plagioclase, however, containing fewer of these than the grano- 

 phyre variety. When the latter occurs with its own outlines, 

 as it occasionally does, it appears to be nnstriated, though fre- 

 quently in Carlsbad twins. After hematite, apatite, leucoxene, 

 and little plates of muscovite or kaolin, are the most com- 

 mon inclusions of both varieties of feldspar. In no case could 

 crystals be found fresh enough to yield measurements of suffi- 

 cient accuracy to determine their true nature. 



The quartz is in irregular areas filling in the interstices between 

 the other constituents, and is also intergrown with red feld- 

 spar as has already been described. It contains numerous fluid 

 cavities with little dancing bubbles, and also inclusions of a 

 dust-like substance and little areas of red feldspar. 



The chlorite owes its origin principally to a formerly exist- 

 ing biotite. It occurs both in little radiating spherulites 

 crowded close together, and in plates enclosing quartz and 

 feldspar. Calcite, rutile and leucoxene are its most common 

 inclusions, while the little pleochroic halos* (Hofe) character- 

 istic of this mineral when derived from biotite, are not rare. 



* For the discussion concerning the nature of these halos, see Neues Jahrb. f . 

 Min., etc., 1888, i, p. 165. 



