228 Bulletin of Laboratories of Demson University. [Voi. xii 



A part, at least, of the mica is clearly secondary, while some of 

 it is yet doubtful as to origin, whether primary or secondary. 

 Its general appearance and association in the sections might 

 very well indicate secondary formation for all of it. 



Phenocrysts of both quartz and feldspar occur in well-de- 

 veloped idiomorphic forms, usually in rectangular and squarish 

 cross sections. In the thin sections studied, phenocrysts of feld- 

 spar are more abundant than quartz ; and while the porphyritic 

 texture is poorly developed in the hand specimens, it is very 

 pronounced in the thin sections. The quartz phenocrysts show 

 irregular fractures free from impurities, strained shadows, and 

 occasionally inclose grains of feldspar. The porphyritic feld- 

 spars show in part broadly twinned bands of plagioclase, and 

 untwinned orthoclase. These are frequently rendered nearly 

 opaque from innumerable, closely crowded, dark inclusions not 

 identifiable, along with minute spangles of colorless mica. Zonal 

 structure is rarely observed, and cleavage is usually wanting. 

 Around the borders of several of the feldspar phenocrysts 

 slight embayments, produced by incipient resorption, are no- 

 ticeable. 



Several of the sections were so cut as to include areas of 

 the black pencils which characterize the rock, megascopically. 

 These are distinguished, microscopically, froni the white por- 

 tions of the groundmass only by a distinct medium-to-dark yel- 

 lowish-brown staining, somewhat resembling that of limonite 

 stain frequently observed discoloring tiny areas of the rock, de- 

 rived from the partial leaching of any iron-bearing constituent 

 in igneous rocks. No definite source of the staining was en- 

 tirely indicated in any of the sections, but the areas clearly rep- 

 resent percolation of solutions of manganese and iron salts 

 through the rock. Why the definite arrangement into long 

 pencils and dedritic forms manifested megascopically, evidence 

 is again lacking, for the textural relations of the minerals in the 

 discolored areas are precisely the same microscopically, as for 

 other portions of the rock. The character of the staining sug- 

 gests that the spotted and streaked appearance of the rock is a 



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