19".] IDDINGS— PROBLEMS IN PETROLOGY. 295 



Magma solutions possess different degrees of heterogeneity as 

 shown by the composition of various bodies of igneous rocks. In 

 some there are shght differences in different parts, extending through 

 large masses. In others marked diff'erences occur within short dis- 

 tances in small masses. Variability in the composition of igneovis 

 rocks from place to place is a universal characteristic, resulting in 

 series of varieties of composition within single bodies, and among 

 different masses. The aggregate of all such series of variations in 

 one region may form a continuous series of wide extent; or there 

 may be gaps in the series in one region, which may be filled by the 

 phases of composition exhibited by rocks in another region. 



In one region the composition of a nearly homogeneous rock mass 

 of considerable magnitude may assume a certain local petrographic 

 importance, while in another region it may appear only as a facies 

 of another rock body. There appears to be no chemicophysical rea- 

 son for the production of a magma solution of one mixed composition 

 rather than of another very nearly the same. But it is known that: 

 magmas of intermediate, or more mixed, compositions, are more 

 abundant than those of extreme, or simpler, compositions. 



The accumulated evidence of chemical analysis, microscopical 

 study of rock sections, and observation in the field, shows the exist- 

 ence of wide serial variations of composition, continuous along 

 numerous lines, owing to the number of variable components. This 

 evidence also shows that there is no one definitely composed magma 

 solution more abundant throughout large areas of the earth than 

 others ; none that deserves special consideration, or may be recog- 

 nized as a universal type. It is true, as already remarked, that in 

 certain regions there are large bodies of rock having nearly uniform 

 composition that assume local importance, and serve as types for 

 reference in particular regions. But it must be admitted that the 

 idea of type is subjective, inherent in the petrographer, not the rock. 

 And when all known series of igneous rocks are treated as products 

 of chemicophysical reactions universal in their application, the for- 

 tuitous character of the chemical composition of particular bodies of 

 erupted magma becomes apparent, and the significance of such local 

 types disappears in a systematic treatment of the whole body of 



