Paraiiiorpliic Alteration of Pyroxene. — Gordon. 41 
intergrowths of liornblcnde with other minerals the last men- 
tioned ari^umcnt for tlic paramorphism of compact hornblende 
from pyroxene does not seem to him to be sufficient. ... In 
cases where augite is surroimded by or appears to pass into 
compact hornblende and neither mineral exhibits its charac- 
teristic crystal outhne in any part of the rock under investig^a- 
tion and the rock is unaltered the primary or secondary na- 
ture of either mineral may be questioned : for each mineral may 
be the result of t!ie primary crystallization of the once molten 
magma from which either of the two may separate before 
the other, or either may be the result of the alteration of the 
other, since the change of compact hornblende to compact 
augite occurs in Hie rocks already described." From the study 
of the synchronous development of various rock-making min- 
erals in pumiceous glassy lavas it is evident that caution must 
be used in referring occurrences of parallel intergrowth to 
paramorphic changes. 
Lawson* refers frequently to the presence of a lighter col- 
ored core in the hornblende as evidence of derivation from 
augite, while Winchellf has described with a like interpretation 
occurrences of dark green hornblende surrounded by or inti- 
mately intergrown with colorless portions, the former being 
by him considered as occupying the space of the original augite. 
and the colorless portions as having formed at the same time 
but entirely free from the influence of the p.ugite. This con- 
troverts \\'illiams' view that these so-called zonal hornblendes 
are the efifect of dynamic action, as also that of \'an Hise that 
they represent secondary growths. 
Brietly summarized the evidence commonly adduced for 
asserting the derivation of compact hornblende from augite is 
as follows : 
1 . The zonal arrangement of hornblende around augite 
and the presence of cores or fragments of augite in hoinblende 
individuals. ,(Hawes, Irving, Van Hise, Lawson. Winchell) . 
2. The intimate intergrowth of the two minerals (Wil- 
liams) . 
3. The imperceptible gradation into each along their com- 
mon boundary ( Williams ) . 
• Lawson, A. C, ilnn. Report Geol. Surr. Can., New Ser., vol. iii, part 1 . 
p. 125F. 
t Wi.NCHELL, N. H., Geological Survey of Minnesota, vol. v, 583. 
