200 HENRY S. WASHINGTON 
mass was in motion, and hence before solidification—an objection 
which is fatal. 
That resorption of the hornblende or biotite substance may, 
and does, take place is not to be denied. The occurrence of 
hornblende and biotite crystals with embayments and irregular 
outlines, but with no alteration border, shows that here the 
resorbed substance has been carried off. These cases are exactly 
analogous to the corrosion phenomena seen in other minerals. 
It is indeed hard to explain why hornblende and biotite alone 
should be so favored as to have their substance and form left to 
them in the majority of cases, while quartz, feldspar, olivine and 
augite should lose part of their substance on corrosion so that we 
do not meet with a zone of different composition from their crys- 
tals or the groundmass. 
The egirine mantles about many quartzes in the basic rocks 
may occur to the reader as an exception. But this exception is 
apparent only because the two cases differ radically in kind. In 
the case of hornblende the change takes place within the original 
crystal space and is not due to a chemical combination of horn- 
blende and groundmass substances; while in the case of quartz 
the change takes place outside the crystal in the groundmass and 
is essentially in the nature of a chemical reaction between the 
quartz and some of the molecules of the groundmass. 
As a further objection may be mentioned the fact that, while 
the alteration in general starts from surfaces in contact with the 
magma, yet in some cases it is seen to proceed along cracks 
which, as fat as canbe seen, are untouched by ita) tie wanst 
objection made is, however, so difficult of explanation that it is 
perhaps needless to bring up more. 
And after all, when we come to examine the current theories 
we find them rather vague and unsatisfactory. It is easy to dis- 
miss the subject with the explanation that resorption of the horn- 
blende and subsequent recrystallization of its substance as augite 
and magnetite has taken place, but it is difficult to realize how 
such a process takes place in actual fact with the results that we 
see. It is hard to see why the original substance alone should 
