CHEMICAL COMPOSITION IN SEDIMENTS 471 
origin is here indicated by three chemical criteria and may therefore 
be regarded as well established. 
Analysis No. 4 shows the composition of a gneiss from the Great 
Falls of the Potomac near Washington, D. C. ‘This rock was de- 
scribed by George H. Williams’ and was believed by him to be of 
sedimentary origin. He says: “.... the analysis... . has no 
relation to any known igneous type, but agrees quite closely with 
certain siliceous sediments; so that, so far as the chemical evidence can 
be relied upon, we may safely regard the rock as of sedimentary 
origin.”’ 
The analysis of this rock shows neither a dominance of MgO over 
CaO nor of K,O over Na,O. ‘There is also no excess of alumina 
above the amounts common in igneous rocks, less than 1 per cent. of 
corundum being present in the norm. ‘The amount of quartz present 
in the norm is 52.14 per cent. This quartz percentage might be 
somewhat suggestive of sedimentary origin if supported by other 
criteria, but taken alone has little critical value for the reasons given 
on p. 468. This rock falls in class I, sub-class I, order 3, rang 2, and 
sub-rang 4, of the quantitative system, a subdivision numbering 16 
other representatives in Washington’s tables. In the opinion of the 
writer this analysis affords no valid evidence of sedimentary origin. 
SUMMARY AND CONCLUSIONS 
The chemical analysis, while in some cases of no critical value, is 
in many other cases a valid means of determining whether a foliated 
rock is of sedimentary or igneous origin. 
The utility of chemical data depends upon the facts that the 
chemical characteristics of greatest critical value are developed in 
the belt of weathering during rock disintegration and decay, and that 
in very many of the igneous as well as the sedimentary rocks the 
chemical changes during the development of foliated structure are 
relatively slight. 
In so far as igneous rocks have been affected by the processes of 
weathering either before or after the development in them of foliated 
structures, they tend to approach the sedimentary rocks in composi- 
tion, and the criteria outlined in this paper become invalid. 
« Fifteenth Annual Report, U. S. Geol. Survey, p. 670 
