THE ROLE OF POSSIBLE EUTECTICS IN ROCK 



MAGMAS. 



The appearance of Professional Paper No! i8, The Chemical 

 Compositioji of Ig?ieo7(s Rocks, Expressed by Means of Diagrams, by 

 Professor Iddings, completes a trilogy of works ^ which will leave 

 a permanent impression upon the nomenclature and progress of 

 petrography. No one who has not done a little work of the sort 

 can begin to conceive of the amount of labor employed in the 

 comparatively small paper of less than one hundred pages which 

 has just appeared. And even so, it is probable that vastly more 

 in the way of experiment in trying to group the facts in different 

 arrangements has been done, of which no trace remains. We 

 see the finished railroad survey, and can form some conception 

 of the engineering, but we cannot know how many lines may 

 have been run through the dense forests before settling upon 

 the final track. 



It is easy also to notice possible improvements in the track 

 when it is once completed; and if I make some suggestions 

 along this line, it is only with the greatest respect for the distin- 

 guished authors of the new chemical classification. Whatever 

 modifications the future may have in store, I think that there 

 will be much of their work which will endure, and that such 

 terms as "persalic" and "dofemic" will after a little become 

 household words to the petrographers of all countries. I think 

 the remark of Mr. Iddings, that we must look upon a rock as the 

 chemist looks upon a solidified mass of mixed salts, that "we 

 must think of the study of igneous rocks, their magmas and rela- 

 tionships, as purely physico-chemical problems, involving the 

 measurement and comparison of mass and force, and their definite 

 quantitative expression," as one of very great importance. When 

 he goes on to say that there are no recognizable groupings of 



' The Quantitative Classification of Jgntous Rocks, by CROSS, Iddings, Pirsson, 

 AND Washington, and Professional Paper No. 14 by Washington, being the two 

 previous works. 



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