THE MINERALOGRAPHY OF - THE FELDSPARS 197 
It seems to the writer that the analogy is not perfect. Metal- 
lography deals with the constitution, crystallization, and micro- 
scopic textures of metallic alloys and their relation to their physical 
properties. Such study must necessarily be guided by constant 
. references to the thermo-equilibrium diagram of the system under 
consideration. In the examination of opaque minerals, such as 
the sulphide ores, the diagrams are not at hand, and not until then 
should the term mineralography be employed. Furthermore it is 
questionable whether it is best to limit mineralography to the 
opaques as contrasted with the transparent minerals. Mineralogy 
and petrography become mineralographic when the phase rule is 
considered.and the interpretation of mineral compositions and rock 
textures studied in the light it sheds upon them. ‘This paper is 
an attempt to discuss the essential nature and relationships of the 
feldspars, together with an explanation of the methods used in 
studying them. Discussions of some phases of this subject have 
appeared in German, but no adequate study of the entire system 
has been attempted in this country. The following brief summary 
indicates the substance of the discussion, though not the precise 
order of presentation. 
Many of the principles that apply to metallography and metals 
apply also to minerals. One of these essential principles is that 
of solid solutions. To handle this principle the method known to 
physical chemists as the phase rule has been applied to metal- 
lography, necessitating the construction and interpretation of 
thermo-equilibrium diagrams to set forth the constituents and the 
processes which form metallic alloys. 
_ Now the principle of solid solutions is applicable also to minerals, 
and is indispensable to a thorough knowledge of their composition. 
The mineralogist and the geologist should therefore avail themselves 
of the phase-rule method, and should understand the use of its 
diagrams if they would thoroughly grasp the nature of the con- 
stituents and of the physical phenomena which have given them 
their rocks and minerals, the end products of such processes. 
This study involves: (1) the construction of thermo-equilibrium 
diagrams; (2) the interpretation of such diagrams; (3) the applica- 
tion of these diagrams to the rock-forming minerals. 
