Gabbroid Rocks of Minnesota. — Witichell 269 
currences where the augite is found to alter, it usually changes 
first to diallage and then to chlorite, with the accompanying 
separation of iron oxide, as will be described later. 
From this study of, the inclusions of labradorite and the 
review of other studies, it seems safe to conclude that: 
1. While inclusions of various kinds are very common in 
labradorite, inclusions of no kind are constant. 
2. The black acicular inclusions of various kinds which 
occur so frequently in plagioclasytes and gabbros have no re- 
lation with the iridescence sometimes exhibited by the feld- 
spar, since they occur frequently when no play of colors can 
be seen. 
3. These acicular inclusions are composed of magnetite, 
probably titaniferous, in the vast majority of cases. 
4. The magnetite needles are, in general, primary inclu- 
sions, though they are often accompanied by magnetite un- 
doubtedly secondary in origin. 
5. The peculiar rectangular scales called microphyllites 
and microplakites by Schrauf, are the sole and efifiicient cause 
of the play of colors, since the colors are never seen without 
the microlites, and the latter never occur without producing 
the colors. 
6. The microphyllites and microplakites (probably ilmen- 
ite) of the labradorites studied here do not owe their origm 
to the process of schillerization as described by Judd, nor to 
any process of alteration, but are true primary inclusions. 
Inclusions in the other pjiimary minerals. No inclusions 
of primary origin have been seen in the augite or magnetite ; 
the inclusions which from their color in reflected light have 
given name to bronzite, and which occur quite abundantly in 
the pyroxene of certain other gabbro rocks to be studied, are 
entirely absent in the plagioclasytes. 
Neither have inclusions of indisputable primary character 
been found in the apatite, although a few liquid inclusions 
and several particles resembling magnetite have been ob- 
served which are essentially like those described in the feld- 
spar. This is, however, in apatite, which in spite of its stable 
characters shows marked evidence of decomposition. 
Alteration of the primary minerals. Alteration of the 
labradorite. There are three important methods of alteration 
to be distinsfuished: 
