1 76 The Amertca7i Geologist. September, 1900 
formed gabbro. For the same reason no better material can 
be found for the study of the beginnings of alteration of many 
kinds. 
The labradorite of the gabbro is found occasionally alter- 
ing to calcite, which is usually accompanied by the appearance 
of very finely granular quartz. This alteration is due to the 
action of filtering water, charged with carbonic acid and may 
begin anywhere in the crystal ; once begun it gradually ex- 
tends outward irregularly in all directions. But one center of 
alteration is never much developed before others appear else- 
where in the mineral, to continue in the same way. This sec- 
ondary calcite sometimes contains numerous opaque black in- 
clusions which are so exceedingly minute as to be wholly in- 
determinable. It is supposed that they are magnetite particles, 
and they may represent a part of the material of the numerous 
acicular magnetitic inclusions of the unaltered feldspar. These 
magnetite needles, however, probably contribute a large part 
of the iron entering into the secondary penninite which is often 
formed at the same time. The latter is developed generally by 
the same method as the calcite, that is, it originates ?t various 
points anywhere in the feldspar and spreads irregularly out- 
ward. 
The labradorite often alters to a sericitic mineral which it 
has been impossible to determine absolutely. From the fre- 
quency with which muscovite is found as an alteration product, 
even in basic rocks, it might be supposed to be a true fibrous 
sericite ; but paragonite or even margarite would be a more 
natural decomposition product of a basic plagioclase. The 
optical properties of these minerals are nearly identical and 
when in a finely fibrous condition they can not be distin- 
guished unless they occur in masses large enough to permit 
chemical, or, at least, microchemical examination. The altera- 
tion may commence anywhere in the labradorite, but is espec- 
ially apt to be found where other alteration has already begun. 
It has a marked tendency to develop considerably in a few 
areas rather than form new centers of alteration. This altera- 
tion is called "damouritization" by Lacroix*. 
Flakes of green pleochroic hornblende also occur included 
within the labradorite. They are undoubtedly secondary pro- 
ducts. 
♦Lacroix: Mineralogie de la France. Vol. II, 1897, p. U- 
