THE ORIGIN OF IGNEOUS ROCKS. 151 
the eruptions approached a climax at this center, part of 
them became more and more silicious, passing the general 
mean of 61 and reaching 64, and as an extreme 71.62 per 
cent, of silica; part became less and less silicious and passed 
through 50 to 48 per cent, of silica. It is to be noted that 
the volume of these extreme varieties of lava is much smaller 
than that of the intermediate lavas at this center. 
When the changes in the other essential oxides of these 
rocks are considered in connection with the variations in 
silica, it is evident that as the eruptive action about this 
center proceeded the chemical character of the lavas erupted 
became more and more differentiated. The same can be 
shown for other great series of eruptions; so that it may be 
stated as a general law that the variation in the composition 
of igneous rocks , which constitute a series of eruptions at any 
volcanic center, is the result of the chemical differentiation of 
some intermediate magma. The composition of the inter¬ 
mediate magma may be different in different centers of 
eruption and in different regions, and it will be shown sub¬ 
sequently that the intermediate magma of any particular 
center may itself be the result of a differentiation of a more 
ancient magma or of a primary uniform magma, if such a 
thing can be shown to have existed. 
Having demonstrated by the geological relations of the 
igneous rocks of any one region that their variation of com¬ 
position must be due to a chemical differentiation of an in¬ 
termediate magma, it remains to discover what is the char¬ 
acter of this differentiation and what were the causes which 
brought it about. 
CHARACTER OF THE CHEMICAL DIFFERENTIATION. 
In order to discover what is the character of the differen¬ 
tiation which has produced the variations in the rocks of 
any district, it is necessary to study still more closely the 
chemical character of the rocks. In the tables of analyses 
already presented the original form of the analyses has been 
preserved and the molecular proportions of the essential 
