106 Hl. W. TURNER 
have orthoclase with basic labradorite, although that must be 
rare, or, orthoclase with acid oligoclase. The nature of these 
two rocks would be so different as certainly to make us hesitate 
to designate them by the same name. In the feldspathic rocks 
it seems to me proper to base the classification of these rocks 
primarily on the feldspars, and if we subdivide the feldspathic 
rocks on the basis of the ratio of the alkali-feldspar molecules 
(Or-+ Ab) to the lime-feldspar molecules (An), the true min- 
eral and, to some extent, the chemical relations of the rocks 
will be brought out, and I think more correctly classify them 
than to put the orthoclase, or the alkali-feldspars in apposition 
to the albite and anorthite molecules combined. In order to 
graphically represent the position of the various rocks under 
discussion there is now introduced a table which is self-explana- 
tory. In the column represented in the table we have at one 
end of the series an alkali-feldspar molecule and at the other 
end a lime-feldspar molecule, and the feldspars of rocks may be 
said to be composed of one of these molecules or of isomor- 
phous mixtures of thesame. The rocks at the head of the column 
containing feldspars composed chiefly of orthoclase and albite 
may be designated as orthosite (from the French term orthose = 
orthoclase) when orthoclase chiefly is present; as anorthosite,? 
when anorthoclase chiefly is present, and as albitite when albite 
chiefly is present. The rock at the foot of the column, whose 
feldspathic constituent is largely anorthite, may be designated 
anorthitite. 
It is impracticable at the present time, and, for the purpose 
of this paper, unnecessary to consider the position in this column 
of all the feldspathic granolites; a sufficient number, however, 
are introduced to show the result of the method here proposed, 
as follows: 
Albitite-porphyry or soda-syentte-porphyry.— No. 1521 Sierra Nevada. 
Turner. Seventeenth Ann. Rept. U.S. Geol. Surv., Part I, p.727. Composed 
3The use of this term will be at once objected to by petrographers since it has 
already been used for rocks composed largely of labradorite and more basic feldspars. 
It is a question, however, since the term in this sense is a misnomer, if it would not be 
well to drop it. 
