Herrick et al. on American Norytes and Gabbros. 341 
The relative proportion by bulk of the ingredients as esti- 
mated from measurements of the microscopic section is as follows: 
olivine, 17.36 per cent., bronzite, 20.87 P^'^' cent., magnetite, 6 
per cent., biotite, 2.8 per cent., labradorite, 47 per cent., chlorite, 
2.4 per cent., apatite, etc., 3.57 per cent. These of course are 
empirical and derived from few measurements. The section 
affords beautiful twins of plagioclase with brilliant polarization 
and a tendency to coalesce into aggregates with the axes at right 
angles. The angles noted on either side the twinning plane 
were 14° — 16°; 13" — 14°; 12° — 15°; 15° — 16°; indicating lab- 
radorite. On pulverizing the rock and separating by use of 
sonstadts solution, the feldspar gave a specific gravity of 2.77, 
the slightly excessive weight being apparently due to the suf- 
fusing coloring mattter so characteristic of the feldspars of this 
rock. The pyroxene is very dark brown and affords the w^ell 
developed cleavage lines parallel to i. In cross section the pe- 
culiar cleavage parrallel to / — t is seen as very fine, closely ar- 
ranged lines with inclusions of a black mineral. To this cleav- 
age the extinction is perpendicular, showing the species to be 
orthorhombic, with the characters of bronzite. It is not rare to 
find twins or aggregated crystals with / — ^ as the twinning 
plane. A thin lamina may be interpolated in an otherwise per- 
fect crystal with no obvious rotation of the intei-penetrating 
member. The bronzite is cut by both feldspar and olivine. 
This latter ingredient is in rather large corroded crystals, evi- 
dently at one time of perfect form I, t — z , i — i", O, and was 
the first of the essential minerals to form. Its sections are every- 
where broken by irregular fracture lines which are followed^ 
by inroads of the superficial decomposition, and the outer por- 
tion is frequently altered to a reddish aggregate, but is also us- 
ually enclosed by a considerable zone of bright green chlorite 
with vivid blue polarization colors. The magnetite is found 
b)Oth in regular crj^stals and in corroded irregular masses. The 
magnetite is generally bordered by a zone of reddish matter 
•stained with hasmatite and frequently is accompanied by scales of 
biotite. The chief remaining accessory is apatite in moderate 
abundance. The rock is thus fullv described because of the 
doubt cast ujoon the specific integrity of norytc by many writers, 
