ANCIENT CRYSTALLINE ROCKS-ANORTHOSITE. 
55 
Twinning parallel to both the vertical pinacoids is sometimes seen. The olivine, 
strangely enough, appears to contain less iron than that previously described. 
It is nearly colorless and the cleavage is less perfect. Its decomposition products 
are practically the same, however, and it is still rich in iron. Both pyroxene and 
olivine possess better crystallographic form than in the foregoing rocks. Black 
iron ore and apatite are especially abundant. Large and small grains of beautiful 
deep-red mica are associated with the ore. This facies is an olivine gabbro. 
At the northern limit of the mass the rock is much finer grained, and a sharp 
line between it and the Pikes Peak granite suggests that it is a contact facies. 
Feldspar is abundant in imperfect laths, but is not so plentiful as above. It cor¬ 
responds to the composition Ab 3 An 4 . Diallage is very common in irregular grains. 
It is still more deeply colored than that just described and exhibits a slight pleo- 
chroism in lavender and brown. Besides the titanic oxide thus contained, it is 
frequently filled with several definitely arranged series of minute dark needles, 
doubtless of rutile, the so-called sagenite. Olivine was originally present in large 
amount, but is now wholly altered into the products already mentioned. The 
red mica is rather plentiful, and, as before, always occurs near the abundant iron 
ore, which is probabty titaniferous. Apatite is common, but less so than farther 
from the contact. The texture is granular and rudely ophitic. 
Beginning at the contact and passing to the inclosed segregations one encount¬ 
ers (1) the doleritic contact facies of olivine gabbro, (2) olivine gabbro, (3) olivine 
syenite—the principal rock of the mass, (4) pyroxene syenite, with accessory 
olivine, (5) pyroxene granite. 
This intrusion thus presents a very typical example of magmatic differentiation. 
ANORTHOSITE. 
Closely related to the olivine syenite is a rock which occurs in it as a dike 
10 to 20 feet wide, just inside its contact with the granite. This rock is not par¬ 
ticularly conspicuous, although detached pieces appear on the surface, but it has 
been exposed in several prospect holes. It is nearly white and is composed of 
individuals of plagioclase up to 10 cm. across, which show cleavage and excellent 
albite twin lamellae more than a millimeter thick. The freshest grains have a 
darker-colored, bluish interior and suggest labradorite. A few small grains of 
dark minerals are present between the feldspar individuals. 
The material is too decomposed to permit a determination of the exact char¬ 
acter of the feldspar, but the decomposition products, zoisite and epidote, occurring 
side by side, indicate that it holds considerable lime, and it is probably a calcic 
labradorite. Sericite also results in considerable abundance from the decompo¬ 
sition of feldspar. Small scattered grains of diallage have the same character as 
that in the olivine gabbro. A few fragments of the red mica are also seen. The 
original presence of olivine can not be affirmed, but patches made up of serpentine, 
limonite, and talc (?) probably result from the decomposition of that mineral. 
This anorthosite dike is thus seen to have a mineral composition such that it 
must be considered as belonging to the olivine-syenite group. It is, in fact, an 
extreme facies of the olivine-syenite magma. 
