Kilauea Iki, 1959 — Macdonald and Katsura 
last types, respectively. In 1935, H. A. Powers 
recognized the presence of both these types 
in the Hawaiian Islands, though he did not em- 
ploy the same terminology. Later Macdonald 
(1949^: 88) also pointed out that, although it 
had been thought by Kennedy to be absent from 
the oceanic areas, tholeiite is present in Hawaii. 
In 1955, Powers emphasized the essentially 
silica-saturated nature of the lavas of Mauna 
Loa and Kilauea. Although it had been antici- 
pated to some extent by Powers (1935), C. E. 
Tilley (1950) was the first to definitely point 
out the presence of two distinct rock series in 
Hawaii. These he termed the tholeiitic and al- 
kali olivine basalt series. 
Tholeiite has been defined (Tilley, 1950; 
Kuno et al, 1957) as a rock essentially satu- 
rated or slightly oversaturated with silica in 
which magnesian olivine bears reaction relation- 
ship to orthopyroxene and Ca-poor clinopyrox- 
ene. In contrast, alkali olivine basalt was defined 
as an undersaturated rock in which magnesian 
olivine and Ca-rich clinopyroxene undergo par- 
allel crystallization. The characteristic ground- 
mass pyroxene of tholeiite is pigeonitic, though 
the wollastonite content ranges from about 40 
per cent to less than 10 per cent. That of alkali 
olivine basalt is Ca-rich augite. Hypersthene is 
present in some tholeiites. 
Recently Kuno (I960) has pointed out the 
wide distribution of basalts resembling the cen- 
tral porphyritic type of Mull in their richness 
in alumina, but differing from it in being essen- 
tially nonporphyritic. High-alumina basalt of 
this type has not been found in the Hawaiian 
Islands, though some rocks containing abundant 
phenocrysts of feldspar are moderately high in 
alumina. 
The predominant lavas of the great bulk of 
the visible part of the Hawaiian shield volcanoes 
contain scattered to moderately abundant phe- 
nocrysts of olivine, commonly as much as 5 mm. 
in diameter. These rocks have been called "oli- 
vine basalt” by Macdonald (1949^, b) . Typi- 
cally, however, the olivine phenocrysts are partly 
resorbed, and obviously were reacting with the 
remaining liquid at the time of consolidation of 
the rock. Chemical analyses show many of these 
rocks to be essentially saturated in silica, and the 
pyroxenes are lime-poor. They are thus typical 
359 
tholeiites. They grade into other rocks that are 
otherwise similar but contain normative olivine. 
The extreme of the latter group is picrite-basalt 
of oceanite type, which may contain more than 
50 per cent olivine phenocrysts. These rocks are 
chemically undersaturated with silica, and even 
with the attainment of complete equilibrium 
on crystallization should contain modal olivine. 
As in the more silica-saturated types, however, 
phenocrystic olivine commonly has separated in 
excess of its stoichiometric proportion, and was 
undergoing reaction with the liquid at the time 
of consolidation, as is indicated by their partial 
resorption. The groundmass pyroxene is largely 
or entirely lime-poor augite and pigeonite. Thus, 
although they are undersaturated with silica and 
therefore not true tholeiites, these rocks conform 
with Tilley’s definition of tholeiite as a rock in 
which magnesian olivine bears a reaction rela- 
tionship to Ca-poor pyroxene, and the entire 
group from the nonporphyritic true tholeiites 
to the picrite-basalts of oceanite type may be 
termed the tholeiitic suite. 
Both Powers (1955) and Macdonald ( 1944) 
have pointed out that the variations within the 
tholeiitic suite can be largely accounted for by 
settling of olivine phenocrysts in the magma, 
though movement of minor amounts of pyrox- 
ene and plagioclase also probably are involved 
(Muir and Tilley, 1957; Macdonald, 1949 b: 
1576). Locally differentiation has yielded small 
amounts of iron -rich basalt and granophyre 
(Kuno et al., 1957). 
The major tholeiitic part of the Hawaiian 
shields is succeeded by a relatively small amount 
of lavas of other types, including alkali olivine 
basalt, picrite-basalt of ankaramite type, hawai- 
ite (Macdonald, I960), mugearite, and trachyte. 
This group may be called the alkalic suite, be- 
cause of its relationship to alkali olivine basalt, 
and because the great majority of the members 
of the group contain a larger proportion of al- 
kalies than do members of the tholeiitic suite 
that contain the same amount of silica. The lavas 
of the alkalic suite constitute only a few per 
cent of the total bulk of the Hawaiian volcanic 
mountains. The nature of the transition from 
the tholeiitic to the alkalic suite will be dis- 
cussed in detail in a future report describing the 
results of the recent chemical analyses. 
