(hau.kr.] IHTMORTIERITE. 97 
Microscopically the rock is composed of allotriomorphic quartz, 
allimanite prisms arranged in parallel bands (schistose structure), a 
ittle muscovite, probably secondary, and accessory titanite, titanium 
>xide, rutile (?), pyrite, apatite, zircon, and corundum. 
The sillimanite occurs both as short and long- prisms. The former 
ire usually grouped together in parallel bands which may run across 
he entire slide or may pinch out at both ends, forming a very flat 
ens. The prisms are short and stout, usually more or less ragged in 
mtline, with many transverse fractures. The several parts of a single 
>rism thus broken are at times considerably displaced. Again, an 
entire row of prisms may be broken and displaced. The displacement 
s always in the direction of elongation of the prism. The ends are 
isually more or less rounded, and macropinacoidal cleavage is some- 
hues well developed. Some nearly square cross sections show diag- 
>nal cleavage. Titanium oxide and zircon are at times inclosed in the 
)risms. These prisms occasionally show slight alterations and at times 
;eem to have been entirely removed, forming either an opaque mass 
>r leaving just the outlines of former prisms behind. 
The long, slender prisms are placed at all angles and are not grouped 
n bands as the others are. The edges are sharp and the ends are 
>ften terminated by two faces, forming a spear. These prisms are 
isually smaller than the others and decrease in thickness until they 
ire so narrow as to become entirely opaque. These straight hair- 
ike inclusions were at first taken for rutile or magnetite, but it is 
nought they are merely sillimanite prisms of extreme thinness. 
! Optically the two forms are identical, and are to be classified as 
illimanite. The relief is distinct and birefringence is rather strong, 
phe direction of elongation is always an axis of minimum elasticity. 
the crystals are nonpleochroic and always extinguish parallel. Pleo- 
fhroic halos were not observed. 
The quartz is allotriomorphic and is usually much cracked. In gen- 
ral the principal cracks are parallel to the bands of sillimanite, while 
racks perpendicular to these bands are also numerous. Many trans- 
fee irregular cracks also occur. Some of the quartz grains seem to 
ave been slightly displaced, and the cracks thus formed been rilled 
<ith secondary quartz. Sometimes the bands of sillimanite have 
Iiused "augen" to form. In places the quartz shows large liquid 
elusions, and strings of small inclusions, though not common, some- 
mes occur. Besides the inclusions mentioned and the accessory 
linerals, the quartz contains in places a large amount of very tine 
'paque dust, possibly magnetite. Such sections of the quartz as 
^main dark under crossed nicols show a uniaxial cross with a positive 
gn in convergent light. 
I Of the accessory materials, titanium oxide, probably rutile, occurs 
the greatest abundance. A few grains are probably titanite, but 
Bull. 262—05 7 
