MUSEUM OF COMPAKATIVE ZOOLOGY. 179 



whole grain, containing muscovite, quartz, and albitic feldspar, which 

 mav be itself microcline. 



Figures 5 and G ilkistrate the case of .a clastic feldspar in which the 

 clear feldspar rim ramifies through the grain, entirely crossing it. Fig- 

 ure 6 repi-esents 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. G) polarize in green, while 

 the cloudy portion polarizes in blue, the two parts passing gradually 

 into each other. The chnidy 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 G, 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 gi'ains with high single and double refraction occur, 

 which are probably calcite. The relations of the clear and cloudy fold- 

 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 original 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 albitc type ; that 

 is, it polarizes in an even low tint absolutely without twinning, and 

 contains comparatively large flakes of muscovite arranged parallel to 



