THE ORIGIN OF IGNEOUS ROCKS. 
177 
excess of iron in the form of magnetite and the lime-bearing 
constituents, or it may separate into biotite, x [12 (Mg, Fe) 0, 
6 Si0 2 +(K, H) 2 0, (Al, Fe) 2 0 3 , 2 Si0 2 ], and orthoclase, 
y [K 2 0, A1 2 0 3 , 6 Si0 2 ] plus the lime-bearing constituents, 
with or without olivine, which may be. present in small 
amounts. This interchange of molecules has undoubtedly 
taken place in the extremely differentiated magmas which 
formed the dikes and flows in the Yellowstone Park region. 
A similar development of biotite has taken place in the 
more crystalline forms of all of the other phases of the 
magma in Crandall basin and at Electric peak. Its devel¬ 
opment in the less crystalline lamprophyric varities is due 
to the higher percentage of potash and somewhat lower per¬ 
centage of alumina. It is, nevertheless, in these rocks also 
a function of the degree of crystallization as well as of the 
chemical composition of the magma; consequently the 
characteristic features of lamprophyric rocks are the result 
of the extreme differentiation of a more normal magma, 
and of the physical conditions attending their solidification 
or crystallization. They are not dependent on their occur¬ 
rence as dikes; hence the application to them of the term 
“ ganggesteine ” appears to the writer to be inappropriate. 
These more or less exceptional rocks have the same 
origin as all of the other varieties of igneous rocks con¬ 
nected with one center of volcanic activity—they, are the 
results of the chemical differentiation of a general magma. 
They differ from the commoner varieties by being extreme 
forms of the differentiation. Like them they occur on the 
surface of the earth as extrusive masses, or within the crust 
as intrusive bodies, and in each case they assume different 
mineralogical and crystalline characters, according to the 
physical conditions attending their solidification. The 
physical conditions influencing crystallization, as already 
shown by the writer in another place, differ widely in geo¬ 
logically similar occurrences, but more especially in dike¬ 
like bodies, where the temperature of the surrounding rocks 
is the most essential factor; consequently a classification of 
26—Bull. Phil. Soc., Wash., Vol. 12. 
