740 



REVIEWS 



son were carefully chosen so as to secure typical and unaltered plu- 

 tonic rocks. 



SiO, 



TiOg 



AI0O3 



FcgOg (FeO, MnO) 



MgO 



CaO 



Na,0 



Kgb 



H3O 



P«0. 



Potash Syenite 



60.57 



•53 



15.85 



8.23 



2.59 



444 

 2.13 

 6.02 

 X.06 



Mean of 



3 

 Analyses 



Monzonite 



55-88 



not det. 



18.77 



8.20 



2.01 



7.00 



3-17 



3-67 



1.25 



not det. 



Mean of 

 10 



Analyses 



Mean of 



5 

 Analyses 



Diorite 



56-52 



-25 



16.31 



11.09 



4-32 



6.94 



3-43 



1-44 



1.03 



.40 



Mean of 



16 

 Analyses 



In the syenites the lime is low and the alkalies high ; in the diorites 

 the lime is high and the alkalies low; in monzonite they are about 

 equal. Study of the analyses shows that the monzonites form a chem- 

 ically well-characterized '■'■ Zwischengruppe''' between the syenites and 

 diorites. They are true plutonic rocks of intermediate composition, 

 with a medium lime percentage (6-7 per cent.), about the same amount 

 of total alkalies, equally divided between potash and soda, high alumina 

 (about 17-18 per cent.), and relatively low magnesia contents. 



The structure of monzonite is that of a plutonic rock and is char- 

 acterized by the fact that the orthoclase crystallized last in large allo- 

 triomorphic plates (mesostasis), partially enclosing the other minerals. 

 The latter are plagioclase (usually labradorite or andesite) and augite. 

 Olivine is abundantly present in certain basic members, but lacking in 

 the more acid ones. Biotite, hypersthene, and hornblende occur 

 sparingly. The accessory minerals are titanite, zircon, apatite, and 

 iron ores. 



Outside of the south Tyrol, rocks corresponding to monzonite have 

 been described from Norway, Saxony, Schemnitz, Minnesota, and other 

 parts of the world. An olivine monzonite occurs at Smalingen, while 

 a rock from Rougstock, in the Bohemian Mittelgebirge, is a nepheline- 

 bearing monzonite, and occupies a place between monzonite or augite 

 diorite and theralite. The relations of monzonite to the syenites and 

 diorites are graphically shown by a diagram. Monzonite occupies 



