MUSEUM OF COMPARATIVE ZOOLOGY. 179 
whole grain, containing muscovite, quartz, and albitic feldspar, which 
may be itself microcline. 
Figures 5 and 6 illustrate the case of a clastic feldspar in which the 
clear feldspar rim ramifies through the grain, entirely crossing it. Fig- 
ure 6 represents the left hand middle portion of the grain, which is 
shown entire in Figure 5. The shape is roughly trapezoidal, with an 
irregular edge bounded by the muscovite of the cement. The whole 
feldspar polarizes as a unit, but in different tints. The clear rim and 
the little connecting cross branches (see Fig. 6) polarize in green, while 
the cloudy portion polarizes in blue, the two parts passing gradually 
into each other. The cloudy portion with high powers is seen to be 
filled, as usual, with fluid inclusions, flakes of kaolin and limonitic 
products, which, as seen in Figure 6, are linearly arranged ; in polarized 
light the cloudy material is seen to be arranged in spindle-shaped 
masses. Two black lines which in both figures occupy the centre of 
the clear tongues are aggregates of muscovite, which connect with that 
outside the grain; several smaller tongues of muscovite also run in 
a short distance from the outside. Here and there in the clear feld- 
spar there are isolated large flakes or aggregates of the same mineral ; 
minute colorless grains with high single and double refraction occur, 
which are probably calcite. The relations of the clear and cloudy feld- 
- spar are such that the latter occurs in little isolated areas encroached 
upon by the clear mineral. In one patch only, the clear feldspar shows 
twinning in a few isolated stripes. The cloudy portion shows none 
whatever, and there is no means for determining its origina] character, 
since only small residual patches remain. 
In Figure 4 there is represented a small feldspar which polarizes with 
a low even tint, is clear and glassy throughout, contains flakes of 
brilliantly polarizing muscovite similar to that by which it is sur- 
rounded, and has in general all the characters of the “albitic” feld- 
spars with this exception, that the left hand portion is cloudy ; this is 
due to the same cause as before, namely, fluid inclusions, flakes of kaolin, 
and limonitic masses. 
The feldspar of Figure 3 polarizes in the cloudy portion blue ; in the 
outer clear glassy portion, a red of a higher order, the slide being thick. 
- The cloudy and clear portions have the usual characters ; the latter 
shows here and there a single twin lamella. 
In Figure 7 a small feldspar is represented of the albite type; that 
is, it polarizes in an even low tint absolutely without twinning, and 
contains comparatively large flakes of muscovite arranged parallel to 
