jSo riie American Geologist. November, looo 
rare; the process is always exogenetic, as already described. 
The result of the alteration of the magnetite is limonite; and 
occasional particles having the characteristic color and lustre 
of pyrite can be detected. Both minerals are opaque; pyrite 
has a brass yellow color and bright metallic lustre by reflected 
light, while limonite has a dull or silky, lustre and a grayish 
white color. Pyrite occurs as particles neither fibrous nor 
lamellar ; limonite is rather confusedly fibrous. 
One other mineral occurs which it has been impossible to 
identify. Only three microscopic crystals have been found 
and therefore no chemical examination is possible. Distinct 
crystal outline is common, but unfortunately the only crystal 
of definite orientation has lost nearly all of its outline. An ap- 
])roximate measure gave 119° indicating th- 1 the mineral is 
probably hexagonal. Another crystal of indefinite orientation 
but with sharp outlines is shown in Plate XVII, Fig. 8, with 
the angles as measured. No twinning nor any distinct cleav- 
age has been observed. In a very thick section the mineral is 
yellow with distinct, though weak pleochroism as follows: 
iig = orange yellow 
?/p = honey yellow 
The interference figure from what is probably a basal sec- 
tion is a black cross with no perceptible change on rotation. 
The mica plate shows that the mineral is positive. The re- 
fringence is slightly less than that of apatite. The birefring- 
ence is at least medium strong — the thickness of the section 
prevents a closer estimate. The extinction is parallel or very 
nearly so, and the elongation is positive. 
The mineral occurs in a highly altered area; but its good 
crystal outlines make it probable that it is not secondary; it 
is often immediately surrounded by magnetite and hematite* 
which are in part at least secondary. 
Chemical composition of the plagioclasytk. An anal- 
\sis of the plagioclasytc from Carlton peak, Minn., ga\'e re- 
sults under I. in the table; for purpose of comparison a 
few other analyses are given: 
*In one case apatite encloses all three minerals; but this as- 
sociation with apatite is probably accidental. This occurrence how- 
ever is another indication that the mineral is not secondary. 
