Ixii PROCEEDINGS OF THE OEOLO&ICAL SOCIETY. [vol. Ixxvii, 



stuclviiio: the remarkable series of Tertiarv io-neous rocks for 

 which that region is so justly celebrated. How well he raacle use 

 of that opportunity is shown in his numerous memoirs and papers 

 in the various publications of the United States Geological 

 Survey, especially the great monograph on ' The Geology of the 

 Yellowstone National Park' (1899), and also in the 'American 

 Journal of Science,' the Bulletin of the Philosophical Society of 

 Washington, and other scientific periodicals. His memoir on 'The 

 Eruptive Rocks of Electric Peak & Sepulchre Mountain ' may 

 be taken as an example of his work during this period (Annual 

 Report, 1892), and also his communication to our Journal on 

 'Extrusive & Intrusive Igneous Rocks' (1896), made shortly 

 after his election as Foreign Correspondent. AVork of this kind 

 brought Iddings face to face with the general problem of ' The 

 Origin of Igneous Rocks,' and accordingly Ave have an important 

 memoir by him on this subject in a Bulletin of the Philosophical 

 Society of Washington (J 892). 



In 1895 Iddings left the United States Geological Survey, in 

 order to take up the Professorship of Geology in the University of 

 Chicago. He now became greatly interested in the classification 

 of igneous rocks, and acting in association with Whitman Cross, 

 Pirsson, and Washington, took a large share in devising and 

 elaborating the well-known 'Quantitative System.' This is not the 

 place to consider the merits and demerits of that system, but one 

 statement with reference to it may be confidently made. It has 

 exerted a great influence on petrological thought, and many of the 

 terms introduced b}^ its authors have become definitely established 

 in literature. During the period that we are considering, Iddings 

 paid much attention to the use of graphic methods as an aid to the 

 interpretation of chemical analyses, and devised, among others, a 

 noLethod of expressing in a composite diagram both the composition 

 of individual rocks and the chemical relations of igneous rocks as- 

 a whole. This diagram enables many important relations to be 

 easily apprehended ; for example, it shows at a glance, from the 

 absence of clustering, that it is hopeless to look for any natural 

 classification of igneous rocks based on chemical composition. It 

 was first published, along with others constructed on different 

 principles, by the United States Geological Smwey ('Professional 

 Paper.' ]^o. 18, 1903), and afterwards reproduced in his book on 

 ' Io-neous Rocks.' During this period he also brought out a work 

 on ' Rock-Minerals ' for the use of students, which took the place 



