346 Herrick et at. on American Norytes and Gabbros. ' 
bros^ Diabases and Aiidesytes of JMlnncsota^ has come into our 
hands. Inasmuch as this work touches substantially the same 
ground as that covered by our qotes a few "vs'ords may be necessary 
in further explanation. We differ most radically in the theory 
proposed to explain the origin of diorytes. Believing as we do 
that Dr. Wadsworth has laid lithological students under great 
obligation by calling attention to the genetic relations of various 
species and attempting to base a classification on the actual re- 
lationshijos of the species, we dissent from many of his conclu- 
sions and think his own work abundantly demonstrates that we 
can ill afford at present, to neglect for a nomenclature which 
can only express individual interpretations of endogene modifi- 
cations, the accurate and highly useful mineralogical system of 
Rosen busch, which seemed at one time likely to give us a uni- 
form cosmopolitan terminology. 
We believe field-work will bear us out in suggesting that the 
reaction between diabases and acid country rock is alone com- 
petent to explain the origin of most diorytes. The decomposi- 
tion of diabases in no case so far reported produces a true dioryte. 
Further, it seems probable that the reaction was cotemporaneous 
rather than subsequent. This radical difference is really less 
important as affecting the classification of Wadsworth than it 
would at first seem. We would write, for examj^le. 
\ Hornblendic schist. ^ 
( Noryte ) mica dioryte 
\ Mica schist. ^ (garnetiferous), 
\ Diabase \ mica dioryte 
\ Chloritic quartz schist. *J (calciferous), 
It seems strange that in studying the series of gabbros from 
Minnesota Dr. Wadsworth should have failed to notice the par- 
agenesis of plagioclase. He mentions, indeed, its "replacement by 
ferric oxide, viridite, micaceous minerals, magnetite, orthoclasc^ 
quartz, epidote, etc.," but seems to consider it of no greater im- 
portance than a stage in decomposition. A careful study of 
contact phenomena between quartz-porphyry and diabase por- 
phyryte and olivine diabase at Michipicoten island, confirms 
the belief that the change from anorthite or labradorite to 
orthoclase is a constant symptom of interaction of the kind indi- 
cated above. 
