1 38 The American Geologist. Soptomber, ia9s 
tlie feldspars are filled almost to opaqueness with dust-like in- 
clusions, which, under a high magnification, appear as minute 
crystals whose identity cannot be determined. Small grains 
of pyroxene, olivine and magnetite are also inclosed by the 
feldspar. 
Dr. Bayley (41, Vol. I., p. 700) has shown that the chemical 
composition and the mean density of the feldspar corresponds 
very closely to that of a basic labradorite. The j^lagioclase is 
generally quite fresh. Alteration proceeds around the edges 
and along cleavage and fracture lines. The usual product is 
kaolin, which appears in fine specks. In some grains the 
white alteration product assumes a fibrous form, but the na- 
ture of the mineral cannot be determined. 
Orthoclase occurs sparingly in the gabl^ro at Duluth and 
along its southern limits. It is indicated in the hand speci- 
mens by its red color due to weathering or when fresh by a 
dull white color. The presence of potassium in the rock is 
shown by chemical analyses. 
Diallage is present in irregular, rounded and angular an- 
hedrons often two to six inches across. The color is usually 
dark brown, varying to pink, which variation may be due to 
incipient alteration. The crystal grains are always traversed 
by parallel cleavage lines parallel to the axis c. There is also 
a distinct parting which cuts the cleavage at an angle of 15 
degrees. Pleochroism is absent. Polarization colors are 
high and the maximum extinction angle is 39 degrees, when 
measured from the cleavage parallel to c and about 25 degrees 
when measured from the parting, which is easily mistaken for 
cleavage. The mineral has a strongly developed fibrous 
structure. Numerous tabular inclusions characteristic of 
gabbro diallages occur in many grains. Sometimes they 
form parallel bands crossing the crystals. The diallages are 
usually associated with the augite, olivine and magnetite. 
Hypersthene occurs as small rounded grains in the granu- 
litic phases of the gabbro and as large irregular anhedrons, 
frequently one-fourth of an inch in length, in the coarser 
noryte. The color is black to brown in the hand specimen, 
and light browai in the thin section. Cleavage lines parallel 
to the axis c are prominent. Fracture lines are not numer- 
ous, and when present occur as irregular lines through 
