CHEMICAL COMPOSITION IN SEDIMENTS 461 
is therefore represented by the percentage of corundum in the 
norm. 
Corundum is present in the norm of sor out of the 1,892 superior 
analyses in Washington’s tables. Of the 501, 490 belong to the more 
acid classes I and II. 
The corundum in the norm exceeds 5 per cent. in only 52 out of the 
1,892 analyses, or less than 3 per cent. 
The corundum in the norm exceeds to per cent. in only 12 out of 
the 1,892 analyses, or about # of 1 percent. Of these 12-analyses, 11 
fall in sub-class II of the quantitative system in which the ratio of 
quartz+feldspar+lenads to corundum + zircon is less than 7. Four 
of the 11 members of this sub-class are stated to be possible products 
of contact metamorphism and 2 others are a corundum syenite and a 
corundum pegmatite, respectively. 
Among the 30 pelitic foliates which enter into the average given 
in the table on p. 456 and which are classified in the table on p. 457, 
19 show over 5 per cent. of corundum (excess alumina) in the norm 
and g show over ro percent. Six of the latter fall in the aluminous 
sub-class II of the quantitative system. The average of the 30 pelite 
schist analyses shows 7 per cent. of corundum in the norm and the 
average of the 79 slate analyses g4 per cent. 
Among the pelites and meta-igneous rocks which have been con- 
sidered there is no evidence that the alumina content changes materi- 
ally during the development of foliated structures. It seems safe 
to conclude therefore: 
I. That a sedimentary origin is to be suspected when the analysis 
oj a fresh joliate shows Al,O, in excess oj 5 per cent. over the 1:1 ratio 
necessary to satisfy the K,O, Na,O, and CaO present. 
Il. That when this excess exceeds ro per cent. a sedimentary origin 
is extremely probable. 
CRITICAL VALUE OF THE MAGNESIA-LIME RATIO 
The importance of carbonation as a phenomena of those portions 
of the lithosphere in which solution is active has been fully discussed 
by Van Hise and others. By this process much of the lime and mag- 
nesia present in the silicate minerals of igneous rocks becomes con- 
verted into the more soluble form of carbonates and suffers partial 
