Hawaiian Ferruginous Laterite Crusts — SHERMAN 
321 
TABLE 3 
The Chemical Composition of a Degraded Ferruginous Laterite Profile 
from Lihue, Kauai 
HORIZON 
Si0 2 
ai 2 o 3 
Fe 2 0 3 
Ti0 2 
inches 
per cent 
per cent 
per cent 
per cent 
Former ferruginous crust 
6-18 
28.5 
25.8 
9.9 
24.4 
Former friable layer 
22-30 
32.0 
33.2 
13.4 
7.6 
tible to erosion due to the underlying friable 
layer. Any break in the crust will permit the 
initiation of erosion by the removal of the 
friable soil, thus undermining the hard sur- 
face layer. Eventually fragments of the hard 
surface layer will break off. Erosion will eat 
rapidly into an area and soon only remnants 
of the original hard surface will remain. The 
area found on the white trachyte cliffs of 
West Maui is a good example of this type 
of erosion. 
When the laterite crust is developed in an 
area which is becoming nearly a peneplain, 
that area will undergo degradation. In this 
case the internal drainage becomes restricted 
and the soil solum is moist or saturated with 
water during most of the year. Under these 
conditions the free iron oxides become un- 
stable and are reduced and leached away in 
the sluggish percolating waters. Titanium 
oxide will remain quite stable. The profile 
becomes enriched with colloidal materials 
and silica from the slow-moving ground wa- 
ters of the higher elevations. Thus resilica- 
tion will take place giving rise to a skeleton 
crust which is rich in silica, alumina, and 
titanium oxide and low in iron oxide. A typ- 
ical analysis of a degraded laterite crust is 
given in Table 3. The data show an iron ox- 
ide content of 9.9 per cent and a titanium 
oxide content of 24.4 per cent in the surface 
layer of this former ferruginous laterite crust. 
SUMMARY 
The ferruginous laterite crust has devel- 
oped in three general areas of the Hawaiian 
Islands; namely, the southern and western 
slopes of leeward Kauai; the westerly slopes 
of the main mountain range of Molokai; and 
on the white trachyte cliffs of West Maui. 
The ferruginous laterite crusts are found on 
the long slopes which have a region of very 
high rainfall at the higher elevations and a 
semi-arid condition at the lower elevations. 
The areas of ferruginous laterite crusts are 
located at higher elevations which have a 
definite alternating wet and dry season. 
The ferruginous laterite crust profiles have 
a hard slaglike surface horizon having a very 
high apparent specific gravity. This layer is 
underlain by a friable layer of a thickness 
varying from 4 to 36 inches. This always 
lies over an impervious layer of either rock 
or a plastic clay. The hard surface horizon 
is rich in iron and titanium oxides and very 
low in volatile matter. The friable layer is 
made up of iron oxides, which sometimes 
constitute as much as 80 per cent of the soil. 
A hypothesis is advanced as to the genesis 
of these ferruginous laterite crusts. This pro- 
poses that iron and hydrated titanium oxides 
in the percolating waters from the soils de- 
veloped on the wet areas of the higher ele- 
vations move laterally over the impervious 
subsoil layers and subsequently accumulate 
in the surface horizon by capillary action in 
regions having an alternating wet and dry 
season climate. The hydrated iron oxide and 
titanium oxides are stabilized by dehydration 
and are converted to hematite and anatase 
in the surface horizon. This gives rise to a 
