BYTOWNITE (ABjAN 5 ). 85 



plates in parallel orientation. The plates are tabular in the pinacoid 

 (010) and are bounded by the planes (001), (100), (201), and (101). 

 The angle at which these barbs approach the central part of the 

 feather, though somewhat variable, is approximately 65 in some 

 sections. This suggests the manebach twinning in the portions of 

 the aggregated blades from which such longitudinal sections are cut. 



Referring to the illustrations already mentioned, it will appear that 

 the aggregated blades, which in subparallel bundles form the rays of 

 the spherulite, consist in certain parts of long, flat, twinned lamellae, 

 elongated in the direction of the crystal axis a, each lamella flattened 

 parallel to the second pinacoid (010), the plane of twinning. On 

 both sides of these twinned lamellae are slightly inclined, thin, flat, 

 blade-like lamellae (barbs) which are in double arrangement on each 

 side of the shaft, as shown in cross-section (Plate II). In other parts of 

 the composite blades these doubled barbs appear to be compounded 

 of thin plates flattened parallel to (010) (barbules), the doubling 

 appearing to be due to manebach twinning. In the middle portion 

 of such aggregates there is great confusion of detail, due to wedging of 

 crystals and overlapping within the thin section. Crystallization 

 appears to have advanced from the central portion of such aggregates 

 outward, producing in some places wedge-shaped crystals or wedge- 

 shaped aggregates of parallel tabular crystals, which may behave as 

 a continuous crystal within the body of the aggregate, but may have 

 an outline or surface corresponding to a parallel aggregation of smaller 

 crystals. 



The optical behavior of these feldspar aggregates indicates that 

 their substance is homogeneous throughout, except in several places 

 where the optical properties show that feldspar of another composi- 

 tion has crystallized. These portions are small in proportion to the 

 bulk of the feldspar crystals. They are in several instances fortu- 

 nately cut by the thin section, the two cleavages being almost exactly 

 normal to the section plane. The section is almost perpendicular to 

 (010) and (001). In these feldspars the acute bisectrix is almost 

 exactly normal to the plane of section ; the plane of the optic axes is 

 parallel to the basal cleavage (001). The acute bisectrix is the direc- 

 tion of vibration of the fastest ray, the mineral is optically negative, 

 and the optical properties are those of Ab 4 Ani, as shown in Michel- 

 Levy's diagram.* Apparently the feldspar compound first crystal- 

 lizing was a little richer in calcium than the mixture of the solution, 

 and feldspar of this composition continued to grow until the solution 



* Op. tit., Plate 11. . ... 



