BY HENRY C. RICHARDS. M.SC. 7 



While he does not consider the time yet ripe for a natural 

 classification of igneous rocks, along with Becker he be- 

 lieves that such a classification will be based upon the 

 eutectics occurring in the rocks : that the differentiation 

 of the various rock types from the single parent magma, 

 will be involved, and something similar to the principle 

 of descent used in the classification of animals and plants 

 will be developed. It is interesting to note that closely 

 following on Barker's book, J. P. Iddings. of the University 

 of Chicago, brought out " Igneous Rocks," Vol. I. In 

 many respects the treatment is the same as that of Harker, 

 particularly in those sections dealing with the newer pet- 

 rology, and Iddings, although differing from Harker on 

 the question of rock classification, agrees with him that 

 the existing systems are uns^'stematic, unsatisfactory 

 and confusing. 



Following up the question of rocks classification, we 

 iind Cross, in 1910, in "" Natural Classification of Igneous 

 Rocks," giving an excellent summary of the various classi- 

 fications suggested, and with criticisms of them. It is 

 in the main a defence of the Quantitative Classification 

 originated by himself and three other American petro- 

 logists in 1902. He does not subscribe to a classification 

 by eutectics as advocated by Becker and Vogt, owing to 

 its being extremely hypothetical and based on a part of 

 the rock at best. He also reviews the usual fundamental 

 objections to the systematic use of the factors of mineral 

 and textural characters. Cross states : " It appears that a 

 natural classification of igneous rocks, expressing a relation 

 between their most notable chemical and physical pro- 

 perties and the origin of those properties in the 

 geological occurrence is impossible. The natural 

 history of the objects is too complex. The only 

 remaining basis for systematic classification is in the 

 characters of the objects themselves. The chemical, mineral 

 and textural characters of igneous rocks are each grada- 

 tional as regards several elements. No systematic division 

 can be made except along arbitrary or artificial lines, and 

 in this sense petrographic classification must be unnatural." 



Harker, however, believes we have already the ge»m 

 of a natural S3'^stem in a classification by eutectics. 



