256 
Fig. 1 . Calcareous rhizo-concretions found in soils 
near South Point, Hawaii. Size of concretions can be 
compared with the scale shown in inches. 
layer apparently was deposited around the 
living root and the inner portion was deposited 
after decomposition of the root at a later time. 
The outer layer was comparable in appearance 
to the calcareous layers or sheets in the soil. 
The deposition of the carbonate appeared to 
occur on the surface of very fine texture layers 
occurring in the wind-blown materials, usu- 
ally clay layers. Samples were collected to 
represent the outer and inner portions of 
rhizo-concretions and the sheets. 
The data obtained from the chemical analy- 
sis to determine the composition of the car- 
bonates are presented in Table 1. The total 
carbonate content of the inner portion of the 
rhizo-concretions was found to be a little less 
than 82 per cent, the highest carbonate con- 
centration of the calcareous concretions. The 
total carbonate contents of the outer portion 
of the rhizo-concretion and of the calcareous 
sheets were almost identical, ranging from 65 
to 67 per cent. This would suggest that the 
outer portion of the rhizo-concretions and the 
PACIFIC SCIENCE, Vol. XII, July, 1958 
calcareous concretions were formed by similar 
processes of precipitation. These latter cal- 
careous depositions are dense and hard while 
the carbonate material inside the tubular 
rhizo-concretions is soft and powdery; thus, 
the latter depositions constitute only a small 
portion of the total carbonates. The powdery 
carbonate of the inner tubular structure could 
have been precipitated from the waters carry- 
ing bicarbonates of calcium and magnesium. 
The precipitation by the action of alkaline 
earth elements on ammonium carbonate de- 
veloped by the decomposing organic matter 
as suggested by Mathias (1931) should not be 
discounted. The outer portion of the rhizo- 
concretion and the sheets were probably pre- 
cipitated by the precipitation of particles of 
carbonate near the root or in the more alkaline 
soil layer and the precipitation of bicarbonates 
continues due to increasing alkalinity, a proc- 
ess proposed by Gillam (1937). Similar rhizo- 
concretion formation of pyrolusite has been 
described by Sherman et al. (1949). 
The data in Table 1 show an appreciable 
quantity of magnesium carbonate in these 
concretions. The highest content was found 
in the inner portion of the rhizo-concretions 
which was found to contain 23 per cent mag- 
nesium carbonate, in other words, a little over 
62 per cent of the carbonates was in the form 
of dolomite. The outer portion of these con- 
cretions contained 17.0 and 16.5 per cent 
magnesium carbonate, or in other words, 56 
per cent of the carbonates were in the form of 
dolomite. The calcareous sheets contained 18 
per cent magnesium carbonate, equivalent to 
58 per cent of the carbonates in the form of 
dolomite. Thus, active dolomitization is oc- 
curring in the formation of these concretions. 
The determination of exchangeable cations in 
these soils has revealed a high content of ex- 
changeable magnesium. The high magnesium 
saturation in the exchange complex is the 
result of a high content of magnesium ions in 
the waters which circulate through the weath- 
ing system. This would also give an ample 
supply of magnesium to form the magnesium 
