Great Dyke of Norite of Southern Bhodesia. 
23 
Very small quantities of aluminous pyroxenes and\ 
mica Medium to low 
Moderate to rather small amount of bytownite >- alumina, normal 
Moderate to rather small amount of non-aluminous to high lime. 
monoclinic pyroxene o ' 
Very small amount of the iron ores chromite, pyrite,x 
and magnetite | 
Poorness of iron in the basic silicates olivine, rhombic ILow iron. 
pyroxene, and monoclinic pyroxene 
Eather small amount of olivine ^ 
The differentiation of the magma has resulted chiefly in the separation 
of the lime from the magnesia, the iron accompanying the magnesia, 
whilst the silica content was not so greatly affected by differentiation (it 
became four or five points higher in the femic part of the intrusion because 
of the enstatite). Thus the inner part of the intrusion probably contains 
about 49 per cent. SiOa, about 7 per cent. MgO, and rather more than 
15 per cent. CaO, and the outer portions of the intrusion (pyroxenite and 
peridotite) about 53 per cent. SiO^, more than 30 per cent. MgO, and 
probably less than 5 per cent. CaO. Combining these figures broadly to 
include the whole of this section of the intrusion, the approximate average 
composition as regards silica, magnesia, and lime would appear to be 
respectively 52 per cent., 18 per cent., and 10 per cent." 
* Since this paper was written the Government agricultural chemist, Mr. G. N. Black- 
shaw, has kindly permitted the author to have the figures of three partial chemical 
analyses of Great Dyke rocks made in the agricultural laboratory for the purpose of an 
important investigation of the Great Dyke soils, which is being conducted by Mr. Black- 
shaw. The analyses are here quoted, and for them the author desires to acknowledge his 
indebtedness to Mr. Blackshaw. 
" Fused with Sodium Carbonate. 
Serpentine. 
Enstatitite. 
Norite. 
No. 43-2. 
No. 795. 
No. 1022. 
Per cent. 
Per cent. 
Per cent. 
51-50 
52-70 
46-04 
Ferric oxide and aluminium oxide 
12-85 
15-90 
28-98 
6-70 
0-12 
0-21 
Calcium oxide, CaO 
1-45 
2-90 
14-66 
27-53 
28-52 
7-13 
Soda, etc., not determined." 
No. 432. — From Lalapanzi. A coarse-grained pale sage-green rock made up of 
enstatite largely altered to bastite. The only other constituents noted are a little inter- 
stitial felspar and some iron oxide. A little olivine serpentine may be present. The rock 
is an altered enstatitite. Not examined microscopically. 
No. 795. — From well near homestead on N. St. Quintin's farm, " Cringleford," 
Makwiro. A medium-grained dark olive-green rock made up of enstatite and felspar with 
a trace of monoclinic pyroxene. The felspar, which is interstitial to the well crystallized 
enstatite, is rather too abundant for an enstatitite, and the rock is probably better classed 
