178 BULLETIN OF THE 



inclusions, plates of kaolin, and liraonitic masses, but the boinidary is 

 less distinct against the clear edge, which surrounds only part of the 

 crystal. The niicrocliuo twinning is seen in spots irregularly dis- 

 tributed among areas of single twinning or without visible twiiniing. 

 The twinning runs into the clear feldspar rim a short distance, then 

 dying out; this feldspar of the rim, however, does not stop at the 

 clouded edge, but extends into the clouded portion in irregular areas, 

 which extinguish together. Outside the entire feldspar grain, but often 

 touching it, there are areas of fresh glassy microcline, and also aggre- 

 gates of little clear feldspars in rounded grains of the albite type. Lit- 

 tle tongues of this fresh microcline' penetrate the clastic feldspar grain, 

 and little veins of the albitic feldspar also cross its corners. Quartz 

 grains and muscovite plates also occur isolated in the feldspar ; this 

 grain seems to be microcline associated with glassy microcline and albite, 

 and possessing a clear boundary, which cannot be separated from areas 

 w'ithiu the cloudy portion. 



The second feldspar (No, A), studied in the same way, had a Sp. Gr. of 

 2.578 and indistinct microcline cleavage in the powder. In the slide it 

 shows a faint double (microcline) twinning ; as before, the central part 

 is cloudy with tlie little kaolin or muscovite plates arranged in parallel 

 lines, and also fluid cavities and irregular masses of brown limonite and 

 black iron oxide. The core is surrounded on the two long sides by a 

 clear glassy border of feldspar, about one eighth the width of the entire 

 grain. It has a single multiple twinning, which is parallel to and ex- 

 tinguishes with one of the sets of twins in the core ; the boundary be- 

 tween the rim and core runs in a general straight direction, although 

 jagged in detail. The rim contains flakes of muscovite, droplets of 

 quartz, and little singly twinned albitic crystals. At one end of the 

 grain, but outside, there occurs a large irregular area of doubly twinned 

 microcline, fresh and glassy, and distinct from the cloudy core which it 

 touches, the clear rim being absent. This fresh microcline, which is 

 evidently the same as that found in isolated areas in the rock, con- 

 tains mica and quartz inclusions. It extends as a vein half across the 

 clastic grain, quartz and mica mingling with the microcline. At the op- 

 posite end of the grain a little vein starts from the outside and entirely 

 crosses bntli the clear rim and the core ; it is at first composed of quartz 

 and muscovite, tlicn in the interior it changes to glassy microcline. 



This grain seems to be microcline from the specific gravity and optical 

 tests ; it has freshly formed microcline adjoining it and crossing it in 

 veins ; it has a rim of clear feldspar in optical continuity with the 



