472 EDSON S. BASTIN 
For this reason only fresh foliated rocks should be used in chemical 
studies undertaken for the purpose of determining genesis. 
In the case of foliates of plutonic igneous origin, weathering before 
the development of the foliated structure is believed to have affected 
so small a proportion of the rocks of this class that it is practically 
negligible for the purposes of this discussion. 
Foliates of volcanic origin may in many cases have been subject 
to weathering for considerable periods previous to the dynamic 
metamorphism which developed their parallel structures. In so far 
as extensive weathering has taken place, their differentiation from 
sediments on chemical grounds will be uncertain. It is probable, 
however, that in the majority of cases, weathering in rocks of this 
type has not been sufficient to obliterate their igneous characters. 
Dominance of MgO over CaO is strongly indicative of sedimentary 
origin. 
Dominance of K,O over Na,O is of lesser critical value, but is 
nevertheless suggestive of sedimentary origin. 
The double relationship of dominance both of MgO over CaO and 
of K,O over Na,O affords very strong evidence of sedimentary origin. 
The presence of any considerable excess of Al,O, in the analysis 
over and above the 1:1 ratio necessary to satisfy the lime and alkalies, 
is also suggestive of sedimentary origin. 
High silica content may be indicative of sedimentary origin when 
supported by other criteria. This criterion must, however, be used 
with caution, since silication probably takes place in the dynamic 
metamorphism of certain igneous rocks. 
When three or all of the above relationships hold good, the evidence 
of sedimentary origin may be regarded as practically conclusive. 
WASHINGTON, D. C. 
March, 1909 
