1 74 The Afnerican Geologist. September, i90o 
crystals would first develop, soon to be filled more or less com- 
pletely by liquids, or more commonly by various hydrous 
oxides, derived from the enclosing mineral, except in certain 
exceptional cases, as, for example, those occuring in labra- 
dorite. Rosenbusch and Williams* on the other hand consider 
them to be of primary origin. Bayleyt, studying the gabbros 
of Minnesota, decides that these inclusions are of secondary 
origin and due to the action of schillerization, basing his opin- 
ion on two observed facts, viz : first, the inclusions often occur 
in certain zones which polarize in different tints from those 
shown by the other parts of the pyroxene, and, second, the 
pyroxene immediately surrounding the inclusions "seems to be 
more changed from its original condition than portions of the 
same lamellae at a greater distance from them." These facts 
will be questioned by no one wdio has examined a number of 
thin sections of the rocks in question. But the interpretation 
of the facts is a different matter. Bayley argues that both 
facts indicate that the inclusions in the process of development 
have absorbed some of the material of the surrounding pyrox- 
ene, and thus changed its appearance and optical properties. 
Xow, the lamallse truly polarize in different tints, but that is 
not all — they extinguish at a distinctly different angle. What 
more can be said of true twinning? Furthermore, the twin- 
ning is not inseparably associated with the inclusions; it oc- 
curs where no inclusions can be seen, and on the other hand in- 
clusions occur which cause no such twinning. (See plate 
XVII., figure I.) The association of the twinning with the 
inclusions is not an extraordinary occurrence ; on the con- 
trary it is very often noted in pyroxenes, and is usually at- 
tributed to the fact that the mere presence of such inclusions 
at a time when the rock was under pressure could undoubtedly 
determine the formation of twinning. 
It has been said that the mineral seems to be "changed 
from its original condition" about the inclusions. This is only 
true in the sense that the area immediately around the grains 
is clearer and more transparent than those portions further 
removed. An attentive examination with the highest powers 
will show that the cause of this appearance is that the mass of 
*G. H. Williams: Amer. Jour. Sc. 1886, XXXI, p. 31. 
tW. S. Bayley: Jour. Geol. 1893, I, p. 703. 
