460 EDSON?®S. BASTIN 
CRITICAL VALUE OF THE ALUMINA CONTENT 
The absolute content of alumina in a rock has little critical value 
unless the proportion be strikingly unusual. What is of significance, - 
however, is its proportion in relation to the bases, lime, and the alkalies, 
with which, in rocks, it is commonly combined. The high content 
of alumina in proportion to these bases in the class of pelitic sediments 
is too well understood to require extended discussion here. It is well 
known that this arises from two causes, (1) the tendency toward a 
concentration of alumina in the finer portions, and of silica in the 
coarser portions of soils and other products of rock disintegration; 
and (2) the comparatively stable and insoluble character of alumina 
which leads to an increase in its relative abundance when rocks are 
acted upon by solutions. In the analyses of metamorphic rocks avail- 
able to the writer there is no evidence that the proportion of alumina 
relative to lime and the alkalies varies greatly under conditions of 
anamorphism such as exist during the development of foliated struc- 
tures. The increase on the contrary takes place under conditions 
of katamorphism, especially within the belt of weathering, and is 
frequently manifest in the development of silicates such as kaolinite, 
hydro-micas, etc., proportionately richer in alumina and poorer in 
silica and bases than the more common aluminum silicates of igneous 
rocks. 
In the igneous rocks, lime and the alkalies are the common bases 
with which alumina is combined, usually in the proportions of 1:1 in 
the feldspars, muscovite, nephelite, etc. In most igneous rocks 
alumina is not present in excess of the 1:1 ratio to the available 
lime and alkalies. In others it exceeds this ratio but the excess is 
almost invariably’ small. If in an unweathered foliated rock the 
excess is large it throws doubt at once upon its igneous origin. 
It is a very simple matter to determine the amount of “excess” 
alumina present in all the superior analyses of igneous rocks tabulated 
in Washington’s tables, since in computing the norm of these rocks 
for classification according to the quantitative system, alumina is 
first allotted to K,O, Na,O, and CaO in the proportions of 1:1 
and the excess of alumina calculated as corundum. The excess 
1 Literature summarized in Failyer, Smith, and Wade, ‘‘The Mineral Composi- 
tion of Soil Particles,” Bull. 54, Bureau of Soils, U. S. Dept. of Agriculture. 
