SN eee ere ee 
THE MINERALOGRAPHY OF THE FELDSPARS 279 
in character .. . . [and limited to] a single species .. . . ,a large 
proportion of [which] is frequently untwinned.”” The paramphibo- 
lites derived from the action of granitic intrusives and metamorphism 
upon Grenville limestone ‘are composed of quartz, microcline, 
orthoclase, and plagioclase.’* This selective habit of the feldspars 
is explained on the ground that in the freezing of a magma the 
feldspars “‘split along the eutectic line.” If the feldspar composi- 
tion, in the magma, was on the potash side of the eutectic line the 
resulting crystals would be dominantly the orthoclase type of feld- 
spar, while if it was on the other side plagioclase (plus a little 
potash feldspar) would result. But if the position of the molten 
feldspar was on or near the eutectic line the solid minerals would be 
divided on freezing into orthoclase (carrying a little soda feldspar 
in solid solution) and plagioclase. 
Conclusion.—The criteria may be summed up as follows: 
Sedimentary Origin Igneous Origin 
Original quartz High pyroxene content 
Motley collection of feldspars Evenly “split” feldspars 
How successfully these criteria have been applied to amphibo- 
lites whose origin was not forthcoming from the field relations 
cannot as yet be stated, but hope is entertained that some progress 
has been made in this difficult problem.’ 
APPENDIX? 
THE SOLUBILITIES OF THE FELDSPAR COMPONENTS! 
In order to understand the nature and construction of minerals 
from a mineralographic point of view, it is preferable to commence 
our consideration with reference to the state of homogeneous 
fusion. Although it may be that certain pairs of silicic salts cannot 
tF. D. Adams, ‘“‘On the Origin of the Amphibolites of the Laurentian Area of 
Canada,” Jour. Geol., XVII (1909), p. to. 
2 Since this was in type a recent paper in this Journal on the ‘‘Feldspar Method’ 
of distinguishing sedimentary and igneous metamorphics has appeared. 
3 This section is introduced to furnish the reader who is not familiar with the 
meaning and interpretation of thermo-equilibrium diagrams a simple explanation of 
their construction and value. 
4’The manner of presenting this topic has been patterned very closely after that. 
of Rosenhain (Introduction to Physical Metallurgy, p. 78, 1915). It is of interest to 
note the similarity of mineralography to metallography. 
