576 
PETROLOGY: H. S. WASHINGTON 
being present in these only in traces or at least in very small amounts. 
The few reUable analyses of augites from highly potassic rocks, on the 
other hand, show little soda, but a marked preponderance of magnesium 
over iron. It may be noted, incidentally, that there are no natural 
potassic pyroxenes or amphiboles corresponding to the sodic ones. 
On the other hand in the ferromagnesian micas, biotite and phlogopite, 
the alkali metal is entirely or almost entirely potassium, the amount of 
sodium being generally negligible. In these there is usually a very 
marked predominance of magnesium over iron. Only in the potash 
mica, lepidomelane, which is characterized by high ferric iron, does 
iron dominate over magnesium, but it is to be noted that this variety 
seems to be confined to the sodic rocks. It is also noteworthy that there 
are no true soda micas, except paragonite, which is only known in meta- 
morphic rocks. It may be added that the potassium-lithium mica, 
zinnwaldite or cryophyllite, carries much more iron than magnesium, 
and attention was called above to the fact that lithium is usually asso- 
ciated with sodic rocks. 
Of the multitudinous examples of the general law furnished by igneous 
rocks only a few prominent ones can be given here. 
Highly potassic rocks are very rare, but the few known regions are 
characterized by high magnesium relative to iron. Such are the wyo- 
mingite, orendite and madupite of the Leucite Hills, Wyoming; the 
jumilKte and verite of Murcia, Spain; the leucitic rocks of Celebes 
and of the Gaussberg, in the Antarctic; many of the leucitic rocks of 
the volcanoes of Western Italy (through here the dominance of mag- 
nesium over iron is not so pronounced). There are also many other 
scattering examples of syenites, shonkinites, minettes, leucitites, leucite 
tephrites, and other dopotassic rocks. Indeed the same general rela- 
tion holds good even in such rocks as peridotites, pyroxenites and horn- 
blendites, in which the amounts of magnesimn and iron are very high 
and those of the alkaUes very low. 
Of the converse case, high sodium with high iron relative to magnesium, 
very striking examples are the highly sodic rocks of the JuHanehaab 
District in Southern Greenland, in which magnesium is usually present 
only in traces and often wholly absent; the ijoHtes and other nephehte 
rocks of the Kola Peninsula in Finland; the nephelite syenites and re- 
lated rocks of Ontario, Norway, the Transvaal, Portugal, Brazil, and 
other regions; and the sodic lavas of Pantelleria, Madagascar, and the 
Great Rift Valley in East Africa. Indeed illustrations of the law are 
so abundant among the highly sodic rocks as to make selection difficult. 
In the figure is given the result of plotting nearly two hundred superior 
