THE ORIGIN OF IGNEOUS ROCKS. 
141 
the magnesia. Both of these show a marked decrease in 
amount from the less silicious to the more silicious end of 
the series, while the iron oxide maintains a more uniform 
position, decreasing only slightly, and being considerably 
in excess of the other two oxides in the more silicious rocks 
of the group. 
In the Etna group the most noticeable feature is the quite 
uniformly high position of the lime and iron and the increase 
of magnesia from the less silicious to the more silicious end 
of the series, and the greater variation in the magnesia,. It 
is in general much lower than the other oxides, and the lime 
is in most cases the highest. 
In the silicious group from Pantelleria the lime and mag¬ 
nesia are comparatively constant, the lime being fully double 
the magnesia, while the iron oxide is greatly in excess, and 
increases toward the more silicious end of the series. In the 
last two groups the noticeable decrease in alumina is accom¬ 
panied in one case by an increase in magnesia and in the 
other by an increase in iron oxide. 
It would seem that these illustrations of the different be¬ 
havior of the molecules of the essential oxides in various 
groups of rocks are sufficient to establish the consanguinity 
of these groups upon a chemical basis. It is perfectly evi¬ 
dent that the correctness of such conclusions rests on the 
completeness, and accuracy of the chemical analyses, and it 
will be admitted that the work of the chemists connected 
with the U. S. Geological Survey commands the utmost confi¬ 
dence. Too much stress cannot be laid on the absolute neces¬ 
sity of thorough and correct chemical investigation of the 
rocks of any region where the freshness of the rocks warrants 
the undertaking. Moreover, the worthlessness of inaccurate 
analyses is equally apparent. It is undoubtedly true that 
the confusion brought about by incorrect chemical analysis, 
often of unfit material, has seriously blocked the progress of 
petrology for years, so that the chemical development of the 
science is still in the future. 
The chemical individuality of geographical groups of rocks 
