but many of the staurolite crystals are much larger, reaching 2 cm. 
in diameter and 5 cm. in length. The matrix is seen under the 
microscope to be mainly muscovite in moderately coarse Hakes with 
minor amounts of quartz, chlorite and biotite, Small granules of a 
black iron ore are abundant and occasional very large biotite crys- 
tals. The embedded staurolite crystals show the typical pronounced 
pleochroism from pale yellow (X,Y) to reddish brown (Z), with 
high refractive index and biaxial figure. The smaller staurolites 
(2 mm.) in the section cut are mostly quite free from inclusions, 
but some carry a large number of the black iron ore granules. I he 
total lack of quartz inclusions is unusual, as Van Hise draws atten- 
tion to “the absence of inclusions of the iron-hearing constituents 
of the schists in garnet and staurolite, and the presence of abundant 
quartzose particles.”* 
Maeroscopically the staurolite is seen to be dark brown in 
colour, well crystallised, and of all sizes, from 1 to 20 mm. in diam- 
eter, some crystals being short and stout, others long and thin, the 
faces m 1 ( 1 1 0 ) AVI) 1 ( 010) b 2 are seen on all, and r 1 (101 ) r“ on 
many. Twins on z(232) are fairly common even with the largest- 
crystals. The measured angles bW, nihii 4 , mb 1 ', and r r agree closely 
with those calculated. A cleavage parallel to b is distinct. 
The staurolite is very unevenly distributed through the rock 
and is without definite orientation. 
Regarding the origin of the Mogumber staurolite: This mineral 
is usually developed by thermal metamorphism of a non-ealeareous 
sediment at a high pressure, but comparatively low temperature, 
as indicated by the combined water present. Oliloriloid, a mineral 
very similar in composition and origin to staurolite, is found in 
nature under practically identical conditions. The causes which 
lead to the formation of the one mineral rather than the other have 
not yet been explained, and can only be elucidated by a close study 
of the occurrences of both minerals. Other closely related meta- 
rnorphic minerals are garnet and chlorite. Ann llise says of the 
origin of staurolite (Metamorphism, p. 327) 
“Staurolite is similar in its occurrence to garnet, but. apparently 
requires more intense met amorphic action for it to begin to foim. 
Its most widespread occurrence is in the schists and gneisses of sedi- 
mentary origin. Tt also develops in profoundly metamorphosed 
rocks of eruptive origin, but it is not known as an original constit- 
uent in any eruptive rook, hike garnet, it may be abundantly < en- 
veloped in* the zone of anamorphism in rocks which are cut by m- 
trusives. The conditions favourable to its formation are therefore 
similar to those which produce garnet (see pp. 300-302) and such 
minerals as tourmaline, andalusite, sillimanite, and eyanite, with 
* Treatise on Metamorphlsm, i>. 701. 
