1921.] Thomson.—The Geology of Western Samoa. 63 
water in the upper reaches than near the mouth. The explanation pre¬ 
sumably lies in the abundance of relatively young lavas, which are not 
rendered impervious by weathering and the production of clayey substances 
along their joint-planes, as is the case in older lavas, and which allow water 
to percolate freely owing to the abundance of vertical cracks and horizontal 
hollows which are general features of recently-cooled pahoehoe lavas. 
Volcanic Rock-types and Mineral Resources of Samoa. 
It will not be out of place here to give a list of all the rock-types 
described from the Samoan Islands. 
Savaii: Olivine basalt, olivine - enstatite basalt, olivine tachylite, 
nepheline basanite, phonolite. 
Apolima : Nepheline basalt. 
Upolu: Limburgite, olivine basalt, olivine basalt-porphyrite, trachv- 
dolerite, nepheline basanite. 
Tutuila : Limburgite, olivine basalt, andesitic basalt, spilite, nephe 
line basanite, trachydolerite, nepheline basanite, trachyte, alkali 
trachyte, phonolitic trachyte. 
Aunuu : Trachydolerite. 
Ofu : Olivine basalt. 
Olosega : Olivine basalt. 
Tau : Olivine basalt. 
Previous to Weber’s examination of Friedlander’s material only basalts 
and limburgite were known. The olivine basalts are by far the most abun¬ 
dant rocks, and these are, of course, rocks undersaturated in silica. A further 
degree of undersaturation leads to the development of some nepheline, 
often determinable only by microchemical examination, and the rock is 
then classed as a nepheline basanite or a nepheline basalt. Only a few 
rocks are saturated in silica, the trachytes. As in the Hawaiian Islands, 
the trachytes and nepheline basanites may be regarded as in part compli¬ 
mentary differentiation products of the basalt magma. Chemical study of 
the various rock-types is necessary before this subject can be pursued 
further. Hobein’s analyses of Tutuila rocks, published by Weber, are much 
too incomplete to serve this purpose. 
The presence of trachytes in Tutuila leads one to expect that these rocks 
may be discovered in Upolu also. This may some day have an economic 
significance, for it has been found in Maui, Hawaiian Islands, that by using 
suitable quantities of a trachyte, combined with more basic rocks, cement of 
quite good quality can be manufactured, the lime being obtained from coral- 
reef detritus. The trachytes supply the necessary silica. In vieAv of the 
abundance of limestones and claystones in Fiji, however, it is doubtful 
if Samoa could ever compete with it even in making cement for local use. 
Young basalts and their associates are practically never the country of 
mineral deposits. In Fiji there are large fields of Tertiary andesites con¬ 
taining copper minerals and gold. Similar andesites may underlie the 
basalts of Upolu, and would, if found, be worth searching for minerals; 
but the general similarity of Samoa to the Hawaiian Islands in age, struc¬ 
ture, and types of volcanic rock does not point in this direction. Quartz 
is one of the rare minerals in Hawaii. 
No study of the lateritic soils of Samoa has been made, and typical 
laterites are unknown. It is possible that bauxitic laterites of value as 
sources of aluminium may be discovered, but it must be remembered that 
