MARREX AND PAL.VCI1E. — QUIXCY PEGMATITES. 151 



it usually strongly predominates about the outside of the grains, often 

 forming the entire margin. It is generally very finely twinned, rather 

 high magnifications being required for its clear definition. The twin- 

 ning lamellae are usually broader and stronger in the marginal parts. 

 Thealbite about the margins is often extended out into adjoining feld- 

 spar and (juartz grains, and these parts are sometimes deorientated. 

 Separate small crystals of albite, usually elongate in habit and finely 

 twinned, are commonly found about the larger feldspar grains. These 

 often penetrate into the adjoining crystals sometimes with sharp cr3'stal 

 terminations, particularly with reference to the quartz. With them are 

 quartz and microcline and these smaller, interstitial grains in places 

 form almost a groundmass for the larger crystals. 



The peculiar relations of the albite to the microcline, quartz, and 

 aegirite in the fine-grained rock lying below the central pocket in the 

 Fallon pegmatite has already been described. It is especially note- 

 worthy that albite is lacking in innermost portions of this pegmatite 

 except in this fine-grained rock, and that here it is not in perthitic 

 intergrowth except to a slight extent. 



Inclusions in the Fell spar. — The feldspar of the pegmatite as well 

 as the granite is remarkable for its inclusions. There appears to be 

 a strong tendency toward regularity in their arrangement. Thus in 

 sections they may be seen arranged roughly parallel to the twinning 

 plane, the direction taken by the perthite strips, or to cleavage direc- 

 tions ; again they ai)pear to he wholly without definite arrangement. 

 Aegirite and riebeckite microlites are both usually quite numerous, 

 but their relative proportions vary considerably. It has often been 

 observed that they are apt to be more abundant in the immediate 

 neighborhood of large crystals of aegirite-riebeckite, and that they 

 may in such ca.ses even have an orientation exactly parallel to the 

 large crystal. The riebeckite has the form of minute shreds or 

 rods rarely exceeding 0.02 mm. in width and usually many times as 

 long as wiile. They sometimes form clusters. Strings of small crystals 

 of riebeckite also occur along fractures in the feldspar. In optical char- 

 acter they correspond to riebeckite. Their prevailing color is a rather 

 light to dark blue, often more or less smuky. The aegirite microlites 

 vary much in size from grains whose dimensions are mciisured in 

 thou.sandths to those measured by a few hundredths of a millimeter. 

 Usually with a distinct elongation parallel to the vertical axis, they are 

 often curiiiusly irregular in habit. Sometimes j)articles of ditVeront 

 sizes and .^hapes are arranged in strings following some defniite direc- 

 tion through the feldsjtar. In color they are pale yellowish-green, or 

 yellow to almost colorless when in very small grains, a is always 



