340 
The Americaii Geologist. 
December, J900 
II. Hornblende-gabbro from near Seal Ledge. 
III. Gabbro-dioryte from Green Point.* 
IV. Labradofels from Burnt Head. 
V. Gabbro-pyroxenyte from Burnt Head. 
VI. Diopside from No. V. 
VII. Green augite from the Cape Verde Islands. 
(Praya S-.n Thiago).| 
Si02 
TiO. 
I 
40.61 
.65 
AI,03 25-90 
Fe,03 2.18 
FeO 5-37 
MgO 7-69 
CaO 14-50 
Na.O 2.31 
K..O 25 
Ign 78 
Spec. gr. 
100.24 
. 2.91 
II 
44 
79 
I 
84 
15 
18 
4 
13 
8 
21 
7 
93 
14 
10 
2 
18 
30 
I 
Z?> 
99 
99 
3 
04 
III 
IV 
V 
VI 
VII 
47-2 
45-78 
43-17 
42.55 
43-99 
.84 
1-56 
1.66 
18.64 
30-39 
9-93 
11.72 
14.01 
1.96 
1-33 
8.78 
3-75 
2.09 
6.82 
1 .22 
6.88 
6.74 
8.84 
8.28 
2.14 
6.80 
8.88 
10.88 
11.52 
16.66 
20.96 
22.51 
19.42 
2.91 
1.66 
1-77 
1-55 
1.09 
.28 
.10 
.16 
-05 
1.44 
• 51 
-31 
-49 
99.89 
99-79 
100.30 
99-90 
100.32 
2.02 
2.83 
3-28 
3-36 
The above analyses reveal the general character of the rock 
very clearly. Differences in the chemical composition of the 
various types are quite in keeping with their mineralogical 
fluctuations. Thus we find the hornblende-gabbro (II), rich 
in pyroxene (hypersthene and diallage) and brown hornblende, 
contains more iron (4.79 per cent) and SiOs (4.13), and less 
AhOs ( 10.72 per cent) than the olivine bearing feldspathic 
noryte (I). The high per cent of SiO^ (47-2) in the gabbro- 
dioryte (III) combined with 18.64 per cent AbOs and but 11.52 
CaO may be explained by the presence of some quartz and 
acid feldspar (andesine) in this rock. (See p. 338.) 
The labradorfels (IV) and gabbro-pyroxenyte (V) repre- 
senting the products of extreme differentiation of the noritic 
magma, show the most pronounced chemical differences. Thus 
No. I V,composed almost entirely of bytownite(see An. p. 333) 
has considerable more SiO^, Al^O^, CaO and over 10 per cent 
*No. Ill is a mass analysis of six specimens of gabbro-dioryte 
containing approximately 45 per cent actinolite (uralite) 20 per cent 
bytownite. 5 per cent andesine. 20 per cent primary green hornblende, 
10 per cent magnetite+apatite-|-quartz-|-mica. 
tSee Hinze: Handb. der Min., p. 1112 (CCCVIII). 
