310 J. D. TRUEMAN 
generally gained and: those above lost in SiO, percentage. As the 
preceding diagram is not intended to represent a tabulation of 
facts so much as to indicate one of the probable courses of chemical 
change, it has not been considered necessary to state the references 
for the rock analyses used, though the best available ones were 
employed. 
In regard to other oxides than SiO,, it seems that the platy 
minerals are, in general, characterized by low CaO and Na,O 
content and they are generally high in Al,O, and H,O as compared 
with an average composition of igneous rocks. When these oxides 
are higher than the amount required to form platy minerals one 
might suppose from analogy with SiO, that they would tend to 
become partially eliminated during the development of foliation 
provided that they are in a soluble form. Available analyses 
indicate that such is the case. Further investigations are, how- 
ever, needed before exact figures can be determined. 
Near igneous intrusions the process of chemical change is prob- 
ably generally different from that which takes place during dynamo- 
metamorphism, and the introduction of material from the magma 
may lead to a composition different from that which would be 
expected in the latter case. Possibly the resulting composition 
would be nearer that of an igneous rock in character. 
CAN CHEMICAL COMPOSITION BE USED AS A CRITERION FOR THE 
DETERMINATION OF THE ORIGIN OF FOLIATED ROCKS ? 
The answer to this question can only be satisfactorily deter- 
mined by careful chemical studies accompanied by close field 
observations. It will depend mainly on two factors: first, the 
character of the chemical changes as compared with the character- 
istic differences in chemical composition between igneous and 
sedimentary rocks, and, second, on the extent of the chemical 
change in large bodies of rock. 
Regarding the first, it is to be noted that platy minerals are, 
as a rule, rather low in CaO and Na,O and somewhat high in AL,O,, 
as compared with average igneous rocks. An increase of such 
minerals would seem to lead to a composition similar to the sedi- 
mentary type described by Bastin. An examination of analyses, 
