30 R&R. D. Irving—Origin of the Hornblende of the 
where described them under the name of augite-syenite.* In 
most sections of these rocks quartz is present, and in the more | 
distinctly granite-like kinds may be of a primary nature, but most 
of it is plainly secondary, or at all events has been deposited 
after the crystallization and solidification of the rest of the rock. 
The augite of these rocks is always in very subordinate quan- 
tity; it is nearly always very much altered, the more ordinary 
ate of iets being to a mass of nearly opaque ferric 
xide. some cases, however, the change has been to a true 
bigetditeiicle or uralite. 
Greenstones.—The greenstones of the region in question may 
be divided as to their mineralogical composition into the mie 
groups of peridotite, gabbro, diabase and diorite; and a 
their geological relations, into the three groups of eas. ' 
Huronian and Keweenawan greenstones, 
All four of the forms of greenstone mentioned are met with 
intersecting the older gneisses, much the most common being 
the diabases. The peridotites, since they never have been 
found to carry hornblende,do not need to be considered here. 
Hornblende has, however, been met with in the gabbros and in 
every case yet studied it is plainly derived from augite or dial- 
lage.t The diabases include both olivine-free and olivinitic 
kinds, the sections so far as examined being usually free from 
hornblende, but whenever this mineral occurs it is always 
plainly altered augite. The diorites are not common, the only 
sections thus far examined coming from Rib River in the west- 
ern part of Marathon county, Wisconsin.§ (N. W. 4, Sec. 26, 
T. 29, R.5 EH.) These represent a coarse- rained rock which | 
may be seen macroscopically to be compose essentially of dark- 
green lustrous hornblendes, 8h derge- sized feldspars which often 
show striations. In a tbin n, however, a large number of 
the hornblendes are seen to gant cores of augite or diallage, 
often several cores in a single individual. 
e greenstones or basic massive rocks of the Huronian 
have been studied microscopically in the Marquette and Meno- 
minee regions by Wichman.| I have myself studied them in 
the Penokee region of northern Wisconsin, and in the Thunder 
Bay, Pigeon River region of the north shore of Lake nupeae 
* Third Annual Report of the Director of the U. S. Geological Survey, p. 
so the “ Copper- dag | ta of Lake Superior,” vol. v, Monographie ie - 
the rock is in the ayes composed of feldspars, usually more or less corroded b 
quartz. It grades through semi-porphyritic kinds, which might per erhaps be. called 
grantio.porphyry, into true stash eres But I am reluctant to coin a new 
me" + The term Laurentian is here used provisionally only to cover those gacteess 
and rg which seem sone: ag ‘uae the Huroni 
Geol. of Wis., iv, pp. 701, § Geol. of Wis. . iv, pp. 698, 699. 
Geol. of Wis., iii, pp. 618, array. 
