ORIGIN OF FOLIATED CRYSTALLINE. ROCKS 301 
ing differences between the compositions of igneous and sedimentary 
rocks. His reasons for considering that the composition of a rock 
remains substantially unchanged during the development of folia- 
tion and that, consequently, it can be used as a criterion for the 
determination of the original character of a metamorphic rock are, 
briefly, based on the following: the application of criteria derived 
from normal igneous and sedimentary types to metamorphic 
examples whose origin has been established by other evidences and, 
second, instances in which the chemical compositions of both altered 
and unaltered rocks are known, a dike described by Teall* being 
used as an example. Bastin summarizes his conclusions as follows: 
Dominance of MgO over CaO is strongly indicative of sedimentary origin. 
Dominance of K,O over Na.0O is of lesser critical value, but is nevertheless 
suggestive of sedimentary origin. 
The double relationship of dominance of MgO over CaO and K.O over 
Na.O affords very strong evidence of sedimentary origin. 
The presence of any considerable excess of ALO; 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 sup- 
ported 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. 
It is, perhaps, advisable to mention here that in the case of many, 
possibly in the majority of, igneous rocks either the MgO is in 
excess over the CaO or the K,O over the Na,O, a fact which Bastin 
recognized. The tables of Washington show that in the majority 
of igneous rocks containing over 70 per cent of silica the K,O is in 
excess over Na,O. Indeed it is not till the silica has dropped to less 
than 60 per cent that the dominance of Na,O becomes marked. 
Accordingly, while the double relationship is, apparently, significant, 
the single ratios have but little value. 
For some years C. K. Leith and W. J. Mead, of the University 
of Wisconsin, have had under consideration the possibility of marked 
changes in composition during the development of foliation in rocks. 
_1J.J.H. Teall, Q.J.Geol. Soc., London, XLI (1885), 133. 
