290 IDDINGS— PROBLEMS IN PETROLOGY. [April 21, 



minerals in solution. The problem of the determination of the 

 mineral composition of rocks is for this reason more complex than 

 at first appears, and is further complicated by the difficulty of 

 determining the amounts of colored and colorless crystals, when 

 they appreciably overlap one another in thin section. 



Another obvious result of a comparison of the actual mineral 

 composition of igneous rocks with the chemical composition of 

 their magmas is the notable variability in the combination of minerals 

 that may in some instances result from the crystallization of mag- 

 mas of like chemical composition. This is true both as to kinds and 

 amounts of the resulting minerals. A striking illustration of this 

 variability is found in the mineral composition of three rocks from 

 the same region, the parish of Gran. Norway, which have been 

 described by Brogger. Analyses of the three are shown in columns 

 I, 2 and 3, Table II. The first rock is an essexite, the second a 

 camptonite, the third a hornblendite, and while the compositions 

 dififer slightly in percentages of silica, and to a less extent in other 

 constituents, the chemical resemblances are striking, and the three 

 analyses lie within the range of many well-known series of analyses 

 of particular rocks. 



TABLE IL 



The first rock consists of lime-soda-feldspar and augite, with 

 some olivine and mica, and rarely a little hornblende. The second 

 rock consists of feldspar and hornblende in nearly equal proportions ; 



