PETROLOGICAL ABSTRACTS AND REVIEWS 637 



OsANN, A. ''Der chemische Faktor in einer natiirlichen Klassifi- 

 kation der Eruptivgesteine, II," Abhandl. d. Heidelberger 

 Akad. d. Wiss., Math.-naturw. Kl., Abh. 9, 1920. Pp. 59. 

 In the first part, reviewed above, the plutonic rocks are discussed; 

 in this the extrusives are considered. One hundred and fifty-one types, 

 based on 973 analyses, are given, but owing to present conditions of pub- 

 lication, only the type rocks are shown, and the number of analyses 

 falling in each group is indicated. The tabulation follows that given 

 for the plutonites. 



Patton, Horace B. "Geology and Ore Deposits of the Platoro- 



Simimitville Mining District, Colorado," Bull. ij. Colo. Geol. 



Surv., 1917. Pp. 122, maps 3, pis. 40. 



The region here described lies between Creede, Alamosa, Chama, and Pagosa 



Springs. The rocks which occur are very similar to those described in the 



fohos of the San Juan region. Short descriptions are given of rhyolite, latite 



(two chemical analyses), andesite (one analysis), basalt, monzonite (one 



analysis), quartz-monzonite-porphyry and diorite. 



Petrographic Committee. "Report on British Petrographic 

 Nomenclature," Mineralog. Mag., XIX (1921), 137-47. 

 A committee, consisting of Watts, Elsden, Flett, Teall, Thomas, Tyrrell, 

 Evans, Hatch, Holmes, Prior, Rastall, and Smith, from the Geological Society 

 of London and the Mineralogical Society, report on ninety rock-names and 

 petrographic terms which have been used in more than one sense by British 

 authors, and make recommendations as to those which are to be rejected and 

 definitions of those to be retained. Some of the recommendations are in 

 direct opposition to the recommendations by the Committee on Petrographic 

 Terms of the U.S.G.S. in 1897 and 1898. For example the British committee 

 recommend the use of the terms porphyry and porphyrite in the sense used in 

 Germany, namely porphyry for rocks with dominant alkali-feldspar and 

 porphyrite with dominant plagioclase. The U.S.G.S. recommended that 

 "Porphyry and its derivatives are to be used as purely textural terms, without 

 limitation to mineralogical groups. Porphyry wiU thus apply to all rocks, 

 whatever their composition, containing phenocrysts in a distinct groundmass, 

 and without regard to the size of the grains of the groundmass. Porphyrite 



is discarded as superfluous " Further in regard to the use of the 



hyphen, the U.S.G.S. recommends that only similar terms be hyphenated, thus 

 two mineral terms or two rock terms, as biotite-muscovite granite, granite- 

 syenite, etc., but mineral and rock, or rock and texture, as unlike terms, are 

 not vmited. The British committee say: "When a mineral-name, or names, 



