Gabbroid Rocks of Mvuicsota. — WincJicll. 285 
Peridotvte. 
Periclotyte occurs only rarely in Minnesota and the writer 
has not been able to obtain even a single specimen. It doubt- 
less represents the ultrabasic portions of the gabbro magma 
and is rare in proportion as the plagioclasyte, the relatively 
acid part of the magma is common. Its composition is said 
to be essentially; pyroxene, usually augite, olivine, and a va- 
rying, but very small, amount of labradorite. It is thus a pi- 
cry te. 
Chapter VI. Orthoclase Gabbro. 
This rock was first described by Streng* in 1877 as a horn- 
blende gabbro, since he considered the amphibole to be an or- 
iginal mineral. With this exception, his description is quite 
accurate, though rather summary. 
Irving,! in 1880, pointed out the importance and preval- 
ence of the orthoclastic constituent of the rock, End gave it the 
name orthoclase gabbro; later, het showed that the amphibole 
was often, if not always, secondary. 
Orthoclase gabbro differs from normal gabbro essentially 
in containing a notable amount of monoclinic feldspar. 
It occurs in great abundance near Duluth, and at several 
other points in Minnesota and Wisconsin. It forms at Duluth 
the southern limit of the gabbro area, rising in high hills and 
bold bluffs back of the city. In structure it is massive, and ir- 
regularly jointed; diaclases run in every direction, and the 
faces of the resultant high ledges form all angles with one an- 
other. Besides the jointing, the rock often seems to be rude- 
ly stratiform. It is usually very coarse in grain, and all the 
essential constituents are easily seen with the naked eye. The 
color is variable, depending upon the prevalence of orthoclase 
and hornblende. The prevalent type is of a light gray color, 
but this is changed to dark green when amphibole is abund- 
*A. Streng und J. H. Kioos: Ueber die krystallinischen Gesteine 
von Minnesota in Nord Amerika. Neues Jahrb. f. Miner, etc. 1877. 
p. 31. Translation in nth Ann. Rep. Geol. Nat. Hist. Surv. Minn. 
1883. 
tR. D. Irving: Geology of Wisconsin. Vol. III. 1880. 
JR. D. Irving: On the paramorphic origin of the hornblende of 
the crystalline rocks of the northwestern States: Anier. Jour. Sci. 
1883, XXVI, p. 27: Ibid 1884. XXVII. p. 130. 
