484 PETROLOGICAL ABSTRACTS AND REVIEWS 



HoMMEL, W. " Systematise he Petrographie auf genetischer Grund- 

 lage," Vol. I, Das System. Gebriider Borntraeger, Berlin, 

 1919. Pp. xii+174, figs. 5, pis. 5. 



Hommel proposes a double-barreled system of classification, in that he 

 represents a rock by a formula composed of two parts. The first, which he 

 calls his "Molekularformel," is derived directly from the chemical analysis and 

 represents the molecular proportions of the various oxides. The second, or 

 " Konstitutions formel," represents the percentages of certain minerals cal- 

 culated from the chemical analysis. The percentages of the computed minerals, 

 however, resemble those of the normative minerals of the C.I.P.W. system, 

 since they generally do not represent the amounts actually present in the rock. 

 As in the C.I.P.W. system, the feldspars are calculated from the amounts of 

 K2O, NaaO, and CaO present in the analysis. Orthoclase is considered pure 

 K2O, AI2O3, 6Si02, and all the soda is calculated as part of the plagioclase. As 

 a matter of fact, a considerable amount of the soda may occur in the orthoclase 

 or microcline as lamellar intergrowth, either visible or invisible. Where all 

 is calculated in the plagioclase, the resulting mineral is too near the soda end. 

 Potash in muscovite is disregarded, and the only lime or alumina used for the 

 dark minerals is that which occurs in excess of the requirement of the theoretical 

 feldspars. 



So far as the first part of the formula is concerned, no objection can be made 

 to it, since it is confessedly chemical. It has the advantage of simplicity over 

 the C.I.P.W. system, and can be shown in a diagram. Whether it is better 

 than Osann's system is a question. Certainly Osann's latest modification 

 (Der chemische Fakior in einer natiirlichen Klassifikation der Erupiivgesteine, 

 Heidelberg, 19 19. Review to follow in this Journal) offers a rapid means for 

 finding rocks of similar composition. Since only Part I of Hommel 's book is 

 printed, however, it is possible that similar groups of rocks may appear later. 



Hommel 's system can best be illustrated by an example. The kern granite 

 of the Brocken in molecules, reduced to 100, gives SiOj 80.2, TiOi — , AI2O3 8.6, 

 FcjOj 0.5, FeO 1.5, MgO 0.4, CaO 1.4, NaaO 3.3, K2O 4.1. Hommel's formula 

 for this rock is : 



8o.2'2 4-1 F e nic 2.6 = -i-3i P [ 633013,1)8 $■ 

 1.22 ■- -• 



The first part is the molecular formula and is obtained as follows: The figures 

 80.2 represent the amount of the silica molecules (plus TiOj and P2O5) as com- 

 puted above. The large figures immediately after the sign S represent the 

 K2O molecules; the NajO mol. and the CaO mol. in feldspar being represented 

 by the figures above and below the sign. The CaO figures necessarily cannot 

 be greater than the amount of AI2O3 remaining after enough has been used to 

 satisfy the soda and potash. Excess CaO is added to the mafic minerals as c. 

 The mafic constituents are represented by e for Fe203, / for FeO+MnO, m for 



