BYTOWNITE (ABiANj. 83 



law, and no evidence of variation in optical orientation or zonal struc- 

 ture in any one crystal. The crystals are homogeneous. In sections 

 cut at right angles to an optic axis the plane of the optic axes makes 

 an angle of 65 with the trace of the second pinacoid (010). In some 

 sections there is a remarkable appearance of the twinned lamellae. 

 They appear to be faulted in bands across the tabular crystal, as 

 shown in Plate I. But there is no evidence of dislocation in the out- 

 line of the crystal plate ; in fact, there may be continuous lamellae on 

 both sides of the apparently faulted belt. Close inspection of twinned 

 lamellae shows that the several series of discordant belts do not cor- 

 respond in number or in width of the lamellae composing them, so 

 that they are not displaced, faulted sections of a large multiple twin 

 of feldspar, but independent crystallizations in parallel position. 



The illustration shows a cross-section of tabular feldspar cut at 

 right angles to one optic axis and nearly at right angles to the crys- 

 tallographic axis c. The crystal is tabular parallel to the pinacoid 

 (010) ; the belts of multiple twins, which have the appearance of being 

 faulted, extend at right angles to (010). Their growth appears to 

 have progressed from one side of the tabular crystal to the other, for 

 they are blended with a continuous lamella on one side and exhibit 

 a broken limiting line on the other side, against another continuous 

 lamella. They may represent a coordinated set of prismatic feldspar 

 crystals, elongated parallel to the crystallographic axis c, twinning 

 independently of one another, while thickening in the direction of 

 the b axis. 



Bytownite (AbjAnb). 



(58 a_b ). These sections are from spherulitic aggregations of 

 twinned crystals. The spherulite consists of radiating groups of 

 highly twinned bladed crystals of feldspar, which are nearly parallel 

 to one another within one group. But the different groups stand at 

 various angles to one another. This is shown in cross-section (58 b ), 

 Plate II. The blades are not plane-faced or parallel-faced. They 

 curve somewhat and wedge out abruptly. They vary in thickness 

 from 0.18 mm. to about 0.07 mm. and less. The breadth of the 

 blades varies considerably, averaging about 1 mm The groups of 

 subparallel blades are from 2 to 5 mm. in diameter. In length, as 

 shown in section (58 a ), Plate III, the blades are about 10 mm. long. 



Upon magnifying these blades they are seen to be highly complex, 

 and their outline quite irregular. Cross-sections exhibit multiple 

 twinning according to the albite law, the lamellae being sharply defined 

 in some places and indistinct in others. In thin section there are 



