Gabbroid Rocks of Mhmesota . — W mchell. 225 
tinctly inferior as reflecting surfaces. The only other cleav- 
ages at all common are those parallel to the faces of the unit 
prism, that is, ?n{iio) and /*( 1 10), which with thebase/'(ooi) 
give the fundamental form of the older school of French min- 
eralogists. These two cleavages, while occasionally giving 
surfaces apparently of considerable size, on closer examina- 
tion are found to give faces which are broken up and irregular. 
Further, these faces are dull, and only rarely can reflections 
be obtained from them; when found they are always dim and 
indistinct. The cleavage -'//(no) is slightly better than that 
parallel to /"(no). Only one other cleavage has been observ- 
ed; it is parallel to ^''^(150), and, while rare, and only giving 
small surfaces, the reflections are at least as good as those 
from ;/z(iio), 
Crystallographic study. The value of the cleavage angle 
pg'{QO\)i\{o\o), far from being accurately known, has been 
measured many times by nearly all those who have studied 
the mineral, with widely varying results, which may be most 
briefly expressed as follows, with a few determinations of the 
angles /^'(i 10) A (010), ;«^*( 1 10) A (no) and//(ooi ) A ( 1 10) 
added merely for reference: (see p. 226). 
Labradorite possesses no crystallographic angle easier to 
measure, or better adapted to accurate measurement, than this 
cleavage angle /^' (001) A(oio)? And yet ths variations in the 
observed value exceed 1°, and two original measurements in 
exact accord can scarcely be found in the literature of the min- 
eral. 
The measurements which have been made on the labra- 
dorite from Carlton peak were made with a reflection gonio- 
meter of the type now used in the labratories of mineralogy of 
Paris. (Mallard goniometer.) A hand specimen of the rock 
was broken into small fragments, as often as possible along 
these cleavages. The same hand specimen served for all the 
determinations; in optical properties no. indication of any va- 
riation in the nature of the feldspar could be found; the chem- 
ical composition which corresponds very nearly to the com- 
position expressed by AbaAn* is discussed elsewhere. It has 
been noted that the cleavage /'(ooi) often gives excellent large 
reflecting surfaces, while the cleavage ^'■'(010), while inferior 
to /(ooi), is distinctly superior as a reflecting surface to the 
