INTRODUCTION. 7 



immiscible ; thus zircons separate out in the process of consolidation 

 so early, so completely, and in such minute crystals as to suggest 

 immiscibility. On the other hand, Alexejew reached the conclusion 

 that in all cases where solutions do not react upon one another chemi- 

 cally, they become miscible above a certain temperature. Again, all 

 researches on the genesis of minerals from fused magmas show that, 

 as a rule, the crystals are precipitated from undercooled glasses or 

 from miscible liquids. With some possible exceptions, therefore, 

 which, so far as is yet known, are unimportant, the investigation of 

 liquid magmas reduces to the study of isomorphous mixtures (in 

 which the physical properties are continuous functions of the compo- 

 sition) and of eutectic ones. 



A main aim of lithological studies for many years has been to 

 classify rocks. It is difficult to overestimate the importance to the 

 whole history of the earth of a natural and rational petrological tax- 

 onomy. The earliest classifications were largely chemical. After 

 the introduction of the microscope they became chiefly mineralogical, 

 but with Lagorio's famous paper on the nature of the glass base and 

 the processes of crystallization in eruptive magmas, 1887, chemical 

 considerations again become predominant. In my opinion, classifi- 

 cation of rocks on a basis of composition alone can never be satis- 

 factory or final. It is possible, of course, to classify analyses; but 

 rocks are at least for the most part very variable mixtures, without 

 analogy to definite chemical compounds, and this method thus fails to 

 cover the ground or to reveal the relations of parts to the whole. 



On the other hand, physical chemistry seems to me to open the 

 road to a classification which must be helpful and may possibly be 

 final. Among the ten or a dozen important rock-forming minerals* 



* In Bulletin 228 of the United States Geological Survey Professor Clarke has 

 given an estimate, based on nearly 700 analyses, of the approximate relative 

 abundance of the more important minerals found in igneous rocks and aggregat- 

 ing 94.2 per cent. Adding the more important of the minerals which eluded 

 separate computation in one sum, this table takes the following form, which is 

 very suggestive with reference to important silicate solutions: 



Per cent. 



Feldspar 59-5 



Hornblende and pyroxene 16.8 



Quartz 12.0 



Biotite 3.8 



Titanium minerals 1.5 



Apatite 0.6 



Magnetite, olivine, leucite, nepheline, etc 5.8 



100. 



LIBRARY sqJ 





