Soil Sequences in Hawaii — Sherman and Ikawa 
463 
Fig. 1. The differential thermal curves of ultra- 
basic rocks and the soils weathered from them under 
different rainfalls. 
of the soil is being made on the island of Kauai. 
Included in this study is the identification of the 
conditions which result in widely different first 
products of mineral weathering of rock. The 
rocks considered in this study are from along a 
uniform lava flow of a melilite nepheline basalt 
located in the Wailua Homestead area, and ex- 
posed to a range of rainfall of from 45 to 200 
inches per year. The rocks immediately below 
the soils are weathered, most of them to kao- 
linitic and bauxitic saprolite. Samples of weath- 
ered saprolite can be found which have an un- 
weathered core of the rock. In every case, the 
boundary between unweathered rock and the 
weathered clay is very sharp and abrupt. The 
chemical analyses and mineral identification by 
differential thermal analysis are given in Table 
4 and Figure 1. Where the rainfall is between 
40 and 60 inches, the first product of weather- 
ing is kaolin, having a silica content of 32 per 
cent and an alumina content of 28 per cent. 
Under 120 inches of rainfall, the first product 
of weathering is a ferruginous bauxite contain- 
ing 2 per cent silica, 39 per cent alumina, and 
36 per cent iron oxide. The weathered rocks be- 
tween these two extreme points vary greatly, but 
most of them have weathered to either a kao- 
linitic or bauxitic saprolite. However, they show 
a gradation of mineral weathering in which 
gibbsite increases and kaolin minerals decrease, 
as shown in Figure 1. 
The data obtained from the chemical analysis 
of the rock and its soil product (Table 4) show 
a relatively uniform composition in the rock 
cores. Rock D probably has lost some of its 
silica due to the wetness of its location. The soil 
analyses show the increasing influence of leach- 
ing due to increased annual rainfall. As rainfall 
increases, the silica content of the soils de- 
creases (from 26 per cent to 2 per cent), the 
alumina content increases (from 24 to 39 per 
cent), and iron oxide increases slightly (from 
30 to 36 per cent). The chemical and miner- 
alogical analyses by differential thermal methods 
(Fig. 1) indicate that as rainfall increases, the 
stability of kaolin type minerals decreases and 
the free oxides become the stable minerals of 
the soil. The soil from rock A shows thermal 
reaction for kaolin minerals while the soil from 
rock D shows a trace for kaolin and a strong re- 
action for gibbsite. 
SUMMARY 
The soils of the Hawaiian Islands offer ex- 
cellent opportunities for the study of the effects 
on soil formation of the differential intensity of 
the factors involved in soil development. 
There are sequences of soils which reflect the 
effect of rainfall: soils rich in montmorillonite 
type of minerals develop in areas of low rain- 
fall, kaolin develops where rainfall is moderate, 
and free oxides where the amount of rainfall 
