The increasing number of well studied examples of igneous differentiation and comagmatic regions indicates that a compilation of available data may be suggestive to those who are building petrologic theories on the basis of the idea of differentiation. A fairly satisfactory graph has been found for the concise presentation of the data. On this graph are plotted data for 40 rock-masses in which gradational series can be traced, and about 20 series of less definite connection but strong signs of consanguinity. The several series are widely different, showing that different factors may dominate differentiation in different cases. If this compilation of scattered data can be kept in mind in building a petrologic theory for one particular case, its object will be amply fulfilled.
The diagrammatic presentation of rock series has been attempted by several petrologists.2 Few of these plots are so constructed as to show . . .