468 EDSON S. BASTIN 
inconclusive evidence of silication in metamorphism. ‘The process 
is so marked, however, in the case of the pelites that it seems probable 
it may also assume important magnitude in the metamorphism of 
certain igneous rocks. 
High silica content as a criterion of sedimentary versus igneous 
origin must therefore be used with more reservation than the other 
criteria we have considered, since it may be developed in an igneous 
rock during metamorphism. Used in connection with other criteria 
it may have very considerable confirmatory value. Used alone it is 
of very questionable value. 
As in the case of alumina, it is not the percentage as given in the 
analysis that is significant, but the excess silica remaining after 
silica has been allotted to the bases present in the proportions in which 
it is usually present in the common rock-making minerals. This 
excess silica appears as quartz in the norm when the rock is classified 
according to the quantitative system. 
The amounts of “excess’’ silica characteristic of igneous rocks 
may be inferred from the following comparisons of the analyses 
tabulated in Washington’s tables: 
In class I, only 25 analyses show over 50 per cent. of quartz in the 
norm and only 7 analyses show over 60 per cent. out of the 762 tabu- 
lated. 
In class IT, only 14 analyses show over 30 per cent. of quartz in the 
norm and only 1 analysis shows over 40 per cent. out of the 719 
tabulated. 
In classes ITI, IV, and V, none show over 15 per cent. of quartz in 
the norm. 
As an example of the critical value of the silica content we may 
take the case of a foliated rock which falls in class I of the quantitative 
system and which shows evidence of sedimentary origin, either in its 
magnesia-lime ratio or its potash-soda ratio or both. If in the norm 
of this rock the quartz content exceeds 50 per cent. the evidence of its 
sedimentary origin is greatly strengthened. If the quartz exceeds 60 
per cent. the evidence of sedimentary origin is even stronger. 
It is to be noted that among the 30 pelite schist analyses classified 
in the table on p. 456, two are so quartzose as to fall in order 2 of class II 
where the ratio of quartz to feldspars in the norm is>3. There are 
