DIORITE FROM CATHEDRAL ROCKS. 
127 
Cathedral Rocks . — Of the specimens from Cathedral Rocks, the rock 715 of the 
‘ tongue ’ or ‘ pipe ’ east of the shoulder E 2 presents noteworthy features and has the 
characters of a basic diorite or hornblende-gabbro. It consists (Plate IX, Fig. 2) of fairly 
idiomorphic crystals of labradorite, showing symmetrical extinctions of about 22°, and 
large ophitic plates of hornblende, with interstitial quartz in quite subordinate amount. 
The hornblende has pleochroism from brownish-yellow to deep reddish-brown like that 
of barkevikite, but the extinction in some crystals is as high as 15° ; it encloses small 
altered felspars and much apatite. The section shows also two plates of hypersthene, 
one enclosed in hornblende and the other altering to fibrous actinolite ; it gives straight 
extinction and is faintly pleochroic, from pale-green for vibrations along the length to 
pale-pink for those across. Other specimens (712 and 72 L) from the same ‘ tongue ’ 
show similar ophitic plates of hornblende, but contain also some biotite and orthoclase 
and much more quartz. They approach, therefore, more closely to hornblende-granites. 
The hornblende, too, in these specimens is of somewhat different composition, showing 
pleochroism from dax - k-green to brown. Prisms of allanite occur in these rocks and 
apatite is very plentiful. 
A chemical analysis of specimen 715 gave the following result under I : — - 
I. Ia. II. III. 
(Cathedral Rocks.) Mol. ratios. (Hurricane Ridge.) (Sweet Grass Creek.) 
Si0 2 
= 
53 -0G 
•884 
53-71 
53-48 
Ti0 2 
= 
1-60 
•020 
0'74 
1-07 
AIA 
18-65 
•183 
18-00 
19-35 
FeA 
= 
1-44 
■009 
3-99 
2-37 
FeO 
= 
7*58 
•105 
4-05 
4-90 
MnO 
= 
0-05 
•001 
0-24 
0-06 
CaO 
= 
OO 
•147 
6-88 
7-55 
MgO 
= 
3-78 
•094 
5-19 
3-67 
Na.,0 
= 
3-20 
•051 
3-50 
4-07 
K.,0 
= 
1-55 
•016 
3-10 
1-41 
PA 
= 
0'38 
•003 
0-38 
0-62 
H..0 at 110° 
— 
0-16 
jO-16 
ILO above 110° 
= 
0-94 
0-55 
(0'80 
SrO etc 0 - 38 
100 -61 
100-33 
99-89 
The Osann formula of this rock is : — 
S 59J5 a 3 Gj fl\\ W 7'61 
which is between that of Osann’s Hurricane Ridge type : — 
S 58'91 *4 C A fit n 0-3 
and his Sweet Grass type : — ■ 
S 60 05 (, 'i C S,\fm W 8 1- 
The analysis of the mica-gabbro from Hurricane Ridge, Yellowstone National 
Park, is given under II,* and that of the quartz-diorite from Sweet Grass Creek, 
Crazy Mountains, under IILf 
* Iddings, Monogr. U.S. Geol. Surv., 1899, No. 32, Part II, p. 340. 
t Clarke, Bull. U.S. Geol. Surv., 1897, No. 148, p. 143. 
