Febeuaey 20, 1903.] 



SCIENCE. 



295 



Chemical Composition of Igneous Bocks 



expressed by Means of Diagrams : Joseph 



P. Iddings, Chicago, 111. 



The diagrams express the molecular pro- 

 portions of the chief chemical components 

 of igneous rocks; the range of their varia- 

 tion ; the gradations of igneous rocks chem- 

 ically between extremes; the grouping of 

 them according to the system of quantita- 

 tive chemico-mineralogical classification, 

 recently proposed by Cross, Iddings, Pirs- 

 son and Washington. The new diagram 

 differed from the old in that, in place of 

 the dots which were distributed in the 

 earlier charts of the author (see Journal of 

 Geology, April-May, 1898, 219), little 

 colored geometric figures were used, drawn 

 by the method of Brogger. Exceedingly 

 expressive pictorial representatives of the 

 chemistry of the eruptive rocks were thus 

 afforded. 



When discussion was called for, it was 

 first directed against the new names sug- 

 gested by the syndicate scheme of classifica- 

 tion, and some exception was taken by G. 

 P. Merrill to the felicity and significance 

 of the ones selected. This was very well 

 met by the authore, who described the pro- 

 cess of evolution through which they had 

 reached the ones of their choice. W. H. 

 Hobbs critically discussed the relative 

 numbers of analyses on which the ranges 

 were established, urging their fewmess in 

 some eases and their abundance in others. 

 He made the point that some indicated little 

 more than specimen analyses. In reply, 

 the authors showed the variety of the rocks 

 where the analyses were few, and their 

 abundance in the cases of the more common 

 rocks. They also described the care with 

 which analyses had been selected. J. F. 

 Kemp spoke of the good results in diffus- 

 ing a knowledge of the molecular propor- 

 tions instead of the percentage composi- 

 tion which would be accomplished by the 



scheme. He instanced the difficulty of re- 

 casting analyses involving the alferric min- 

 erals as the stumbling-block in close work, 

 and preferred percentage statistics of the 

 minerals in the thin sections to the calcu- 

 lated 'norms.' He also felt reluctant to 

 see texture, which is now so important, 

 become so minor a feature. A. C. Gill 

 spoke somewhat critically of the essential 

 significance of 'norms' which are artificial 

 assumptions, corresponding to no mineral 

 in the rock. He spoke of the good results 

 which could be gained by the use of the 

 percentage of silica as a fundamental prin- 

 ciple in arranging cards of analyses, sub- 

 dividing them, after the choice of conveni- 

 ent groups, on the basis of other com- 

 ponents. 



On the whole from the discussion the 

 more vulnerable part of the proposed 

 scheme appeared to be the difficulty of 

 calculating percentages from many anal- 

 yses and the matter of the norms. With 

 the many advantages of another sort and 

 with its definiteness and logical order, all 

 present were impressed. 



The Nephelite Syenite Area of San Jose, 

 Tamaulipas, Mexico: George I. FiniiAY 

 and J. F. Kemp, New York City. 

 The San Carlos Mountains, in the state 

 of Tamaulipas, Mexico, are largely made 

 up of nephelite-syenite. This rock is ex- 

 posed for ten miles along the range south 

 of the town of San Jose. With it are asso- 

 ciated dacite and andesite in the form of a 

 laccolith, and dikes of tinguaite, analcite- 

 tinguaite, camptonite and diabase. The 

 general geology of the San Jose district was 

 given, with a discussion of the field rela- 

 tions of the above rock types. They were 

 described petrographically, and their min- 

 eralogical and chemical relations are treated 

 in accordance with the sjiidicate scheme of 

 classification outlined above. 



