THE MINERALOGRAPHY OF THE FELDSPARS 219 
The question is, is it orthoclase or microcline that occurs in plagio- 
clase? We have learned from our discussion of isomorphism that 
it is probably microcline. 
THE POTASH-SODA FELDSPARS—THE PERTHITE SERIES 
Before the advent of Vogt’s equilibrium diagram and Warren’s 
masterful discussion of the potash-soda series it was held that this 
system was similar to if not: identical with the plagioclase series; 
the two components being regarded as perfectly isomorphous.' 
But the Winchells? question this and say: ‘‘The close relation 
and gradation of optical properties corresponding to a gradation 
in chemical composition which exists in the plagioclase feldspars 
does not exist, or at least, has not been established, in the soda- 
potash feldspars.” Harker? states that “we have to do with 
WO! 5; ee pte’ series, Or- Ab, and Or-An, with a wide hiatus 
in the middle.” 
There would thus seem to be a hopeless difference of opinion 
regarding the potash-soda series. It will be shown later that 
these two extreme views are not as antagonistic as now appears. 
Zaptee? is right when he says that “‘very little has been written 
regarding perthite, or perthitic intergrowth.” 
J. H. L. Vogt’ was the first to give us an elaborate paper 
regarding the alkali feldspars, but his paper leaves many questions 
still obscure. Makinen® has studied the perthites from the peg- 
matites of Finland. But it has remained for Warren’ to sum up 
the status of the perthite series and to discuss it quantitatively. 
While the binary system, potash-soda feldspars, has not as 
yet been investigated thermally there seems but little doubt that 
tP. Macnair, Introduction to the Study of the Rocks and Guide to the Rock Col- 
lections in Kelvingrove Museum (1911), p. 28; J. P. Iddings, “Obsidian Cliff, Yellow- 
stone National Park,” U.S. Geol. Oe conc Ann. Rept., pp. 269-70; A. H. Phillips, 
Mineralogy (1912), p. 408. 
?.N. H. and A. N. Winchell, Elements of Optical Mineralogy, p. 210. 
3 Alfred Harker, Natural History of Igneous Rocks, p. 244. 
4 Carl Zapfee, Econ. Geol., VII, 137. 
5 J. H. L. Vogt, Tschermak’s mineral. und petrog. Mitt. (1905), p. 24. 
6E. Makinen, ‘Die Granitpegmatite von Tammela in Finnland,”’ Bull. 
a’ Comm. Geol. de Finlande (1913), pp. 1-101. 
7 Chas. H. Warren, Proc. Amer. Acad. Arts. and Sci., LI (1915), 125-54. 
