THE ORIGIN OF IGNEOUS ROCKS. 
119 
removes the element of chance and furnishes occasion and 
basis for the search for the laws according to which differ¬ 
entiation has taken place. 
In discussing the tables of atomic proportions he first se¬ 
lects five analyses in which the alkalies are unusually high 
and the bivalent elements are extremely low, and concludes 
that they represent the composition of a magma in which 
the relations of the alkali metals to aluminum and silicon 
are 1:1:2, there being other constituents present in small 
amount. This hypothetical magma he terms foyaite 
magma. Since this magma is known equally well in the 
form of “ deep-seated/’ “ dike,” and extrusive rocks he con¬ 
cludes that it is not capable of further differentiation, or, 
more correctly, it is only slightly inclined to further- differ¬ 
entiation. 
From another group of analyses, which seem to have a 
rather wide range of composition, he derives an approxima¬ 
tion to an atomic proportion of (Na + K + 2 Ca): A1: Si of 
1:1:2, and the magma represented by these analyses is con¬ 
sidered to have this molecular constitution with an excess 
of silicon, and is called the granitic magma. * 
It is assumed that a magma that is capable of separation 
will be more highly differentiated the more advanced its geo¬ 
logical formation, which is explained as meaning that ex¬ 
trusive rocks as a whole are more highly differentiated than 
deeply seated ones. 
Another group of analyses furnishes the basis for a 
granito-dioritic magma, in which the following relation 
obtains: Na + K > Ca <1 4 Ca, and in which there are con¬ 
siderable amounts of Mg and Fe. This comprehensive 
magma produces granitite, amphibole- and augite-granite 
and their modifications, together with syenites and dioritic 
rocks with their extrusive equivalents. Its characters are 
therefore vague. It is considered to consist of a “ metal 
kern,” (Na K) A1 Si 2 , with another, Ca Al 2 Si 4 , besides which 
A1 and Ca occur at times in excess. The large proportions 
of Mg and Fe in this magma is accounted for by the sup- 
