Gabbroid Rocks of Minnesota . — li 'inchell. 273 
this mineral has been noted as occurring rarely by both 
Adams* and Lawson. f 
The alteratio7i of the magnetite is also very constant in 
method. The first stage is represented by a dull earthy black 
substance. This is believed to be merely the amorphous state 
of magnetite, sometimes called ocherous magnetite. Next the 
color in reflected light changes very markedly, becoming dull 
grayish white to yellowish white, with occasional silver white 
submetallic reflections. In spite of its color, it is still perfectly 
opaque; it has become crystalline again, however, as shown by 
its fibrous character. This stage is limonite or common hyd- 
rous iron oxide. At the same time minute particles sometimes 
appear having the characteristic brass-yellow metallic lustre 
by reflected light of pyrite. The black amorphous condition is 
usually found between the magnetite and limonite. The alter- 
ation seems to begin on the periphery and progresses irregu- 
larly inward; the final effect in all cases is a mass of limonite 
crossed in all directions by lines, perhaps representing irreg- 
tdar planes of the magnetite. 
Alteration of apatite. The apatite, which is frcqucntl)- want- 
ing also shows the effect of decomposition. It is, however, a 
very stable mineral, and is not transformed into any other; 
instead of changing to other minerals the apatite is first tra- 
versed by a few large fractures connected by numberless ver>' 
minute cracks; along both appear various inclusions probab- 
ly introduced by the action of water; these are dust-like par- 
ticles apparently of magnetite, minute drops of liquid, and 
flakes of hematite; I the effect of these foreign elements is to 
give the apatite the appearance of a mineral in the process of 
transformation. 
Secondary minerals. The minerals formed by the altera- 
tion of the plagioclasyte include: calcite, penninite, clino- 
chlore, mesolite, scolecite, pseudomesolite, magnetite, hema- 
tite, limonite, and pyrite. 
*F. D. Adams: Report on the geology of a portion of the Lau- 
rentian area: Geol. Surv. Canada. Ann. Rep. Vol. VIII. 1896. p. 
t A C. Lawson: The Anorthosytes of the Minnesota coast of lake 
Superior: Geol. & Nat. Hist. Surv. Minn. Bull. No. 8. 1893. P- 8. 
X\\\ unknown mineral associated, and even included, within apatite 
will bo described later; the association is supposed to be accidental, 
-'ind is not constant. 
