ALTERED SANDSTONES. 



89 



Various other phenomena can be studied in tills sHde: Zoisite, in prisms 

 and granular masses, is developing in some of the clastic grains; triclinic 

 feldspar, in grains and in polysynthetic microlites, is forming in others; 

 and the resolution of quartz as well as of feldspar can be observed. White 

 mica is forming authigenetically, and perhaps also apatite. Decomposition 

 has also set in and the slide contains some serpentine. These phenomena 

 are better observed, however, in other cases. 





Fir. 3. Authigenetic hornblende in altered sandstone, No. 8, Coast Kangea. a. clastic quartz : b, clastic feldspar; c, 

 autbigenetic bornblondo ; d, autbigt'uetic feldspar aggregate; e, clear isotropic uiasa ; s, serpentine. Magnified 170 

 diameters. 



No. 80, New Idria, is a coarse, bedded, dark greenish-gray sandstone, 

 evidently altered, but manifestly a sandstone. Under the microscope the 

 clastic character is clearly visible, the original limits of the grains being 

 defined by irregular streaks of brown or greenish color. Some of these 

 streaks represent decomposed mica foils. In the clear masses separated by 

 the colored streaks are embedded angular and rounded grains, many of 

 which are quartz carrying abundant fluid inclusions, while others are feld- 

 spar. Surrounding these nuclei are aggregates, chiefly feldspatliic, and these 

 aggregates are so related to the nuclei that it is impossible to doubt that they 

 are composed of authigenetic minerals formed at the expense of clastic 

 grains, only the central portions of which remain. Many of the residual 

 grains are almost entirely surrounded by elongated microlites in positions 

 nearly normal to the surface of the nucleus. The ends of the microlites 

 do not inerel}' abut against the nucleus, but penetrate it for a sensible dis- 



