42 The American Geologist. •^"'>' ^^^'■ 
4. The presence of a common twinning plane (Williams) . 
5. Differences in the coloration of hornblendes; colorless 
within with dark green zone without (Lawson) ; dark green 
within surrounded by a colorless zone or the two intimately in- 
tergrown ( Winchell ) . 
With the exception of the last all the phenomena thus far 
considered may be paralleled in the igneous rocks in circum- 
stances which leave no doubt of the primarv character of the 
hornblende as shown by professor Iddings in the rocks of Elec- 
tric peak and Sepulchre mountain. There is no reason to 
suppose that a like process of synchronous development may 
not have taken place in the recrystallization of the gneisses and 
other metamorphic rocks. In support of this is the occurrence 
described by the author* where the augite and hornblende ap- 
pear as independent growths. Their idiomorphic form and 
their relations to each other and to adjoinmg minerals sug- 
gest independent growth. As against the hornblende being 
of magmatic origin we may note (i) its occurrence in the 
rock which has suffered most from dynamic forces, and (2) 
the absence of idiomorphic hornblende in the least altered 
rock. While it may be assumed that the elements of horn- 
blende originally crystallized as augite it is evident that the 
change here is not one of paramorphism, but perhaps a more 
or less complete dissolution of the augite and a recrystalliza- 
tion of both minerals. 
Whether or not the conclusions in the above cited cases are 
correct does not row concern us. In many and perhaps in all, 
the inference seems a reasonable one, but it remains an infer- 
ence only, the evidence offered lacking the elements of con- 
vincing proof. Nor is such readily forthcoming owing to the 
lack of criteria whereby cases of derivation may be clearly 
distinguished from those due to synchronous growth as Id- 
dings has pointed out. 
There is one line of evidence, however, that may be regard- 
ed as incontestable, viz., the presence of hornblende bands 
bordering irregular fractures in the augite. In the author's 
study of the syenite-gneiss of Canada such cases were ob- 
served under such circumstances as to make the conclusion 
•Syenite-Gneiss (Leopard Rockt fi-om the Apatite Regiim of Ottawa 
Coun, Canada. Bull. Geol. Soc. of Am., vol. vii, p. 118. 
