84 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



opaque lines between blades. These are very irregular and appear 

 to be impurities introduced into spaces between the surfaces of blades 

 during the grinding of the section. The surfaces of blades are com- 

 posed of crystal faces at various angles which are the terminations 

 of component lamellae. A blade so delimited in cross-section is trav- 

 ersed in places by distinct, straight lamellae, which extend without 

 interruption from one side of the blade to the other, as shown in 

 Plate II, On both sides of such a twinned belt the blade is composed of 

 shorter lamellae which appear to originate near the middle of the blade 

 and run outward, their cross-section being like a curved wedge whose 

 apex is at the middle of the blade. In other places there are parallel- 

 edged lamellae in two bands standing at a slight angle to each other. 

 These lamellae appear to blend or to be interwoven in the middle of 

 the blade. They are not twinned in some cases, but are twinned in 

 others. Each of these lamellae terminates at the surface of the blade 

 as an independent crystal, so that the surface consists of the angular 

 terminations of these crystals. In some places they terminate in 

 a common plane. As they do not always exhibit uniform optical 

 behavior, they appear to overlie one another in thin section as 

 inclined plates or prisms. 



In longitudinal section (5S a ) parallel to the rays of the spherulite 

 the long shafts of feldspar exhibit very delicate feather-like structure. 

 This is very intricate and is often blended to such an extent that its 

 precise character is obscure. It appears differently according to the 

 position in which the groups of bladed crystals or aggregations have 

 been cut by the section. In some positions of the section the shaft of 

 a "feather" consists of long, narrow, very sharply defined stripes of 

 albite twins (Plate III), which are clearly longitudinal sections of the 

 well-defined bands of albite twins observed in cross-sections of the 

 blades. This shaft tapers gradually toward the apex. On both 

 sides of this shaft are long, straight-edged lamellae, which make an 

 angle of 4 or 5 with the twinned shaft, and farther out toward the 

 apex an angle of 7 . They appear like barbs in a feather. In some 

 places, noticeably toward the apex of the feather, the barb-like parts 

 are crossed by delicate parallel lines, like barbules. 

 . The position of the twinned lamellae parallel to (010) and the devel- 

 opment of the pinacoidal cleavages parallel to (010) and (001) which 

 appear in cross-sections (58 b ) show that the feather-like blade is 

 elongated in the direction of the crystal axis a, and is broadened par- 

 allel to the basal pinacoid (001). Some longitudinal sections parallel 

 to the second pinacoid (010) show a feather-like arrangement of some- 

 what curved branches or barbs, each composed of extremely thin 



