8 
MUSEUM BULLETIN NO. 2, 
The following table shows the chemical relationship of the 
hypersthene gabbro to other gabbros: — 
1. 
2. 
3. 
4. 
SiOa 
50 36 
49-50 
49-38 
51-68 
TiOa 
0-90 
0-84 
M9 
13-63 
18-00 
18-55 
13-88 
FeiCh. 
2-22 
2-80 
2-06 
6-59 
FcO 
8-38 
5-80 
8-37 
4-44 
MnO 
0-20 
0-12 
0-09 
MgO 
8-67 
6-62 
5-77 
7-87 
CaO 
11-50 
10-64 
9-72 
10*99 
NajO 
2-54 
2-82 
2-59 
2-93 
KiO 
0-75 
0-98 
0-6* 
0-81 
HiO+ 
0-05 
0-74 
HaO- 
0-71 
P 2 0 6 
0-07 
0-28 
0-16 
S.G 
99-98 
2-970 
98-40 
98-56 
99-93 
1. Hypersthene gabbro of Purcell sills. 
2. Average analysis of all gabbros excluding olivine gabbro. 1 
3. Average analysis of all norites. 2 
4. An analysis of Karoo dolerite (olivine diabase). 3 
Variations in the Gabbro . — The most common variation in 
the gabbro is the occurrence of the hornblende gabbro, which is 
believed to be formed from the hypersthene gabbro by meta- 
morphism. In this gabbro, the pyroxenes are entirely absent 
and as a femic constituent occurs a fibrous hornblende with the 
characteristic pleochroism, bluish green parallel to c, strong 
green parallel to b, and pale yellowish green parallel to a. Also, 
these hornblendes contain inclusions of magnetite in irregular 
forms. This hornblende is identical with that described above 
as being formed from augite and hypersthene, and, hence, is 
considered secondary. In the hornblende gabbro, quartz is 
more abundant and with micropegmatite occasionally occurs 
as interstitial material. Epidote and calcite are present as a 
product of deep-seated metamorphism of the feldspars. An 
analysis of this variety is given by Daly , 4 who considers the 
hornblende as primary. 
3 R. A. Daly, Pro. Am. Acad, of Arts and Sci., vol. 45, 1900, p. 211. 
2 R. A. Daly, Pro. Am. Acad, of Arts, and Sci., vol. 45, 1900, p. 211. 
3 Hatch and Corstorphine, Geol. S. Africa, p. 232. 
4 R. A. Daly, Amer. Jour. Sci., 4th Ser., vol. 20, 1905, p. 193. 
