and the Results of its Alteration. 266 
structure are common. ‘The latter forms are shown in fig. 16; 
the fibers here are much more irregular and coarser than is gen. 
erally true. (Compare also fig. 19.) The fibers are apparently 
rounded but the outlines are usually indistinct, and the form 
can be made out only by repeatedly changing the focus of the 
microscope. The explanation of all these irregularities in out- 
line is given by the result obtained on examining the sections 
cut transverse to the fibers. Several additional facts were 
brought out in the study of the sections now described. It was 
found that, when examined between crossed Nicols, the extine- 
tion of the light took place parallel to the length of the fibers ; 
moreover, the fibers have not infrequently a transverse fracture, 
probably indicating cleavage. The form of the terminations of 
the needles could not be certainly observed. In cases like 
those above described (fig. 15), the extremities seem to be 
given entire, but no absolute assertion can be made in regard 
them. In many cases, probably the majority, they taper out 
gradually to a fine point, while in others they seem to be ter- 
minated by a low pyramid. 
The examination of the other set of sections, cut across the 
fibers, was even more satisfactory and conclusive. The appear- 
ance in polarized light, as the plate is revolved on the stage of 
the microscope, is at once striking and beautiful. The section 
as a whole is divided into irregular patches (albite), changing 
from dark to light and the reverse with the revolution, giving 
the whole a strangely mottled look. Distributed closely an 
17. 18. 
tee * g Ys 
uniformly through this matrix are seen also minute areas of an- 
other substance, sometimes curved but generally bent at an angle 
of 60° or 120°; they are wnchanged by the revolution between 
the crossed Nicols. The effect will be best appreciated from 
the accompanying sketches (figs: 17, 18). When a high power 
1s employed (say 600 diam.) and the attention is confined to a 
small portion at once, it is seen that these narrow bands, which 
