2 JOURNAL OF GEOLOGY— SUPPLEMENT 



The Alkaline Rocks 



Nephelite Syenites and Related Types 



Leucite-bearing Rocks 



Escape of Thermal Waters 



Other Conceptions of the Alkaline Rocks 

 Is Basaltic Magma the Parental Magma of All Igneous Rocks ? 



General Considerations 



The Average Igneous Rock Compared with the Parental Magma 



The Earlier Stages of Igneous Rock Evolution Not Revealed 

 The Normal Line of Descent 



Variations from the Stated Scheme of Crystallization 



Monomineralic Rocks 



The Order of Intrusion 



Chilled Border Phases 



The Role of Assimilation 

 Summary and Conclusion 



INTRODUCTION 



With the recent appearance of the books of Harker, Iddings, 

 Daly, and von Wolff, it is natural to expect that the subject of our 

 present knowledge of the igneous rocks and their origin has been 

 thoroughly stated. Nevertheless, it has seemed to the writer that 

 in these treatments of the subject, largely from the point of view of 

 the field geologist, some questions have been left open which are 

 capable of definite solution if the bearing of recent experimental 

 work on these subjects is appreciated. Much of the experimental 

 material was, indeed, not available when these books were in 

 preparation. An attack on the problems of the igneous rocks 

 from this latter point of view has led to a conception of their 

 origin which is stated in the present paper. It is not hoped that 

 the brief statement offered can possibly be the whole truth or that 

 erroneous conclusions have been entirely avoided in a subject of 

 such magnitude and complexity. Some hope is entertained, how- 

 ever, that the discussion offered may have a tendency to turn 

 petrologic thought in a definite direction, and, perhaps, may suggest 

 subjects of experimental investigation which may be pursued with 

 likelihood of finding useful application of their results. 



Differences in composition in associated rock bodies and in 

 various parts of a single body have been referred by petrologists 

 to two principal processes: assimilation of foreign material and 

 differentiation. According to individual opinion, one or the 

 other has been favored and some have proposed processes involving 

 a combination of the two. A consideration of field facts, especially 



