F 
e 
ANORTHITE-ROCK. 59 
Anorthite-rock—At several points on the north or Minnesota shore of 
Lake Superior, between the mouth of Split Rock River and the Great 
Palisades, and again in the high point near the mouth of Temperance 
River, known as Carlton’s Peak, are to be seen exposures of a very coarse 
light-gray to colorless or white rock, occasionally with a faint greenish 
tinge. This is seen in the thin section to be composed exclusively, or nearly 
so, of anorthite feldspar. Often there is no other mineral present except in 
exceedingly minute inclusions, and these are very sparse. In one section 
a few grains of altered olivine were noticed within the anorthite, and in two 
or three a little augite between the feldspar grains. The feldspar appears 
in every case to be anorthite. In no section did it show the peculiar ar- 
rangement of needle-like inclusions met with in European gabbros, and so 
common in the coarse gabbros of Lake Superior, to which this rock is very 
nearly related. 
This anorthite-rock presents very interesting occurrences, as described 
in a subsequent chapter. It appears both as masses cutting black gabbro, 
and as included angular masses in the same rock. 
Tabulation of observations on anorthite-rock. 
Minnesota. light-gray. 
Angle between 
maximum ex- 
tinctions of 
adjacent hemi- 
4 tropic bands of 
5 the plagioclase 
a2 Pla FI Macroscopic char- | Constituents as determined by | 12 sections cut 
g cst 3 acters. microscope. at random in 
5 8 : the zone O; it. 
9 % & . 
oO a Pn es Z 
& £1814! 
5} es |8/e 
a =} $|° g 
n CP ln|aA] & 
729....| North shore Lake | SW.| 5 | 54|8W.| Very coarse-| Pure anorthite, without acces- | 28 30 58 
Superior; 14 grained; the sory. 37 41 78 
miles below crystals reach- 35 41 76 
mouth of Split ing an inch in 
Rock River, lengthand 
Minnesota, near breadth; color- 
center of less to white; 
occasionally 
with a greenish | 
tinge. | 
746....| North shore Lake | SW.| 5 | 54] 8W.| Very coarse-| Pure anorthite, without acces-| 34 40 74 
Superior; 1} grained; the sory. 22 25 47 
miles below single crystals : 29 24 53 
mouth of Split often reaching 
Rock River, 4 inch across; 
