Her rick et at. on Ar/ierican Norytes a7id Gabbros. 345 
are full of inclusions and the latter is nearly always associated 
with magnetite in corroded grains. The plagioclase and dial- 
lage are so fresh that it is believed that these accessories can 
not be the joroduct of internal decomposition. The almost com- 
plete limitation of garnet to metamorphics is additional reason 
for the inference. The present writers, unfortunately, have 
had no opportunity to personally examine this localicy, hence 
can bring to bear no topographical evidence. An examination 
of a number of so-called quartz-diabases and garnetiferous dia- 
bases has shown them all to be diorytes, or easily explained as 
a mechanical mixture due to the explosive dis-memberment of 
a granite when embraced in a dyke-mass. Instances of this 
sort are very well seen in Michipicoten bay where a diabase 
dyke not infrequently contains in itself scattered fragments of 
the adjacent granite, the fragments being of all sizes and ex- 
hibiting in a beautiful way the contact phenomena where heat 
out of the presence of vapor is the sole agent. A specimen 
now before us might easily be called a cpiartziferous diabase 
had we not traced its relations as indicated beyond peradventure. 
The naming of such a chapter of accidents tends we think to 
confuse and retard our progress toward a classification which 
shall be natural in the sense of expressing the relationships and 
development of the rocks.' 
Since writing the above the valuable paper by M. E. Wads- 
worth entitled Preliminary Description of the P eridotytes ^Gab- 
J Irving (Copper-bearing rocks of lake Superior) states that "the distinc- 
tion between diallage and augite is a valueless one, since not only are 
both often found in the same section but every gradation is found in 
rocks of this class from augite to diallage. " 
To this it may be replied that while it is doubtless true that such tran- 
sitions occur, that fact does not obliterate the value of discriminations 
based on two minerals so distinct when typical, or of varieties of rock so 
unlike in appearance and occurrence as diabase and gabbro. It might be 
urged with equal truth, because augite and hornblende pass into each 
other by alteration and both occur in the same rock, that diabases and 
diorytes should not be distinguished. It is customary to divide the rocks 
of the lake Superior region into original and detrital. The term "origi- 
nal" seems misleading, for it is certain that a large part of the basic and 
acid eruptives are paragenous in the sense defined above and arc quite 
distinct from the autogenous members, which alone deserve to be called 
original, though both are undoubtedly eruptive in origin. 
